The 2-Degree Limit IV

Debates Over the 2-Degree Celsius Limit

So we’ve just looked at the history of how the 2-degree limit in the mean surface temperature increase compared to pre-industrial level became the politically agreed, but also the legal objective of global climate change mitigation as a result of an interesting scientific but also political process through the negotiations. Even though that is the politically agreed goal and even though almost all countries are part and parties to the U.N. Convention on Climate Change, it does not mean that everybody agrees with the 2-degree limit. There is in fact a pretty hot debate as to whether this is the right objective for our mitigation efforts. And there are really two sides to the arguments.


So we’re going to try to make sense of these different arguments. On the one side, some leading climate scientists and in particular Professor Jim Hansen we’ve already mentioned quite a few times in these lectures, argue that even a mean surface temperature increase limited to 2-degrees Celsius could be catastrophic. And they instead argue that we should be aiming for a 1-degree Celsius target instead of the 2-degree Celsius target. So that’s one side of the argument. In the meantime some observers claim that staying within the 2-degree limit is way too difficult, but also way too costly if that proved feasible. And therefore, they claim that the 2-degree limit should be either weakened.

I mean we should try to adopt an easier and less costly target or some even go as far as saying that it should be simply dropped and that the world should follow a bottom up approach to climate change mitigation and that after all, countries or businesses should only do what they can do and what they want to do. So that’s the two sides of the arguments. And you can see that the 2-degree limit is pretty much under fire, but does it really mean that the 2-degree limit should be revised?

The problem is that it would be difficult just by looking at these arguments to decide if it should be revised upward or downward. So let’s look in further details at the arguments of both sides to try to make sense of them.

On the one hand, it is completely true that the recent scientific evidence suggests that global warming of 2-degrees Celsius may well generate very severe and irreversible risks. Again, this is why some leading climate scientists are advising for a 1-degree Celsius target instead. What are their arguments, really? Well they point out to the earth’s paleoclimate history. And they say that it, it points to the fact that 2-degrees Celsius of global warming is likely to result in sea level rise of 6 meters in the long-run, which I guess everybody would agree is an extremely significant threat. And it’s very difficult to see how most of the places could barely adapt to such a rise in the level of the sea.

Another argument they have is that they emphasize that global warming of 2-degrees Celsius could induce what they call slope amplifying feedbacks. Another way to call it is to call that chain effects from climate change.

So let’s look very simply at two examples. The first is that the Amazon forests could eventually die off as a result of repeated drought. And this is a very serious potential problem because the Amazon forest is a major sink of carbon emissions. So the die-off of the Amazon forest would increase and release massive amounts of C02 in the atmosphere and that would in turn further aggravate climate change.342

Another example is that the methane and the carbon dioxide currently buried into the permafrost of the tundra could also be released into the air as the tundra melts as a result of global warming. So that’s another example of amplifying feedback and chain effects of climate change.

So it’s true that by pushing the climate beyond the experience of the human era for the past 100,000 years, we risk inducing conditions that are inhospitable for the human species and millions of others by the way. So it’s very important to understand that a 2-degree increase in temperature is far from being risk free. This is not what it is about. But keeping below 2-degrees Celsius of global warming is really absolutely essential to maintain climate change within the boundaries of manageable risks and simply to maintain our ability to adapt to the effects of climate change.

So that was one side of the argument. On the other side, some observers claim that as I already said, staying within the 2-degree limit is too difficult and too costly. Here really the recent scientific evidence does not support this claim. It indicates that keeping below the 2-degree limit for sure is extremely challenging. I mean we’re not arguing here that this is an easy task. But it points to the fact that this is feasible even assuming business as usual, economic growth and development.

There are many global studies, many authoritative global  studies making the same point, including the scenarios reviewed by the IPCC recently released Fifth Assessment Report. But also the publications by the International Energy Agency, the IEA, or the Global Energy Assessment led by the Institute of Applied Systems analysis. All these studies, all these authoritative, serious studies show that reducing global greenhouse gases emissions to a level that is consistent with the 2-degree limit is still within reach, using technologies that are either already commercially available, or demonstrated at the pilot scale.

Even if, and it’s very important to recognize that they would need further research and development to be deployed at the scale that is needed to achieve the deep decarbonization of the energy systems. And we’re going to dedicate one full lecture to discuss the necessary research and development that is necessary to bring these technologies to eventually large-scale deployment. But what these studies also show very clearly is that the window of opportunity to stay, to keep the 2-degree limit within reach is closing very fast. So it, it’s certainly true that countries need to act very quickly and in a very determined and, and also coordinated manner to keep that target within reach, if we don’t want it to simply disappear.

343The question is what would be the cost of reducing greenhouse gases emissions to a level that is consistent with this 2-degree limit? And how does it compare to two things really, one, how does it compare to the size of the global economy? It’s important to look at that, to see if we have the ability to invest into climate change mitigation actions.

But also very importantly, we need to compare the cost of mitigating climate change that is reducing greenhouse gases emissions to the cost of climate change itself because it’s based on this comparison that we should take a decision as to what is the appropriate level of climate change that we can afford and what is the level of climate change we should not exceed because it would induce very high costs. The modeling of the overall impact of climate change in monetary terms, that is if we try to quantify using U.S. dollars for example is a formidable challenge.

And it is very important to recognize the limitations to modeling the world over one century or more. And that clearly demands that we have great caution in interpreting the results coming from these modeling scenarios. That would be very convenient if we didn’t have to use these models, but the problem is that we do. We do, because of the basic nature of the challenge. Because when it comes to climate change, the lags from action to the effect are very long. So the quantitative analysis we need to do because we need to inform policy decisions by quantitative analysis is dependent on these long-range modeling exercises as George Box once very famously said, and there is a lot of truth in that statement, “all models are wrong, but some are useful.”

So we’re going to be using these models with caution, but this is very important that we look at what they have to say to inform our decisions. So what do they teach us? Well with global warming exceeding 4-degrees Celsius, existing models that include the risk of abrupt and large-scale climate change estimates an average 5% to 10% loss in global GDP, the gross domestic product, with poor countries suffering the most in costs in excess of 10% of their GDP.

But there are some limitations as I said to these models. And what we know is that the cost of business-as-usual, so the cost of the continuation of the current trends if we do nothing to control climate change would increase still further if the model were able to take into account more systematically two very important factors which most of the time they fail to do.

The first is if they included the direct impacts on the environment and human health, which sometimes we call the non-market impacts, because they’re difficult to quantify in monetary terms.

And the, the second element is that recent scientific evidence indicates that the climate system may be more responsive to greenhouse gases emissions than we previously thought.

This is what we just discussed, that there might be slope amplifying feedbacks and chain effects that release further carbon dioxide and methane as a result of global warming itself. So if we put these two elements and build them into the modeling framework, then they would increase the total cost of uncontrolled climate change, of business-as-usual climate change to the equivalent of 20% of reduction in consumption per head now and into the future.

344So it is very important to understand that the assessment modeling of the cost of climate change impacts has to be built around the economics of risk and not only the traditional cost-benefit analysis. Because averaging across possibilities conceals the risks. And the risks of outcomes much worse than expected are very real and they could be catastrophic as we have been discussing in the previous chapters. So by nature, climate mitigation policy is about reducing these risks. It’s true that they cannot be completely eliminated, but they can be substantially reduced and this is what we should be aiming for in real life.

So a climate change modeling framework also has to take into account ethical judgment on the distribution of income and how to treat future generations. And it should not focus on narrow measures of income like GDP because as we’ve said, the consequences of climate change for health and for the environment are likely to be severe, but are difficult to reflect into a narrow indicator like GDP. So all these principles need to inform the way we do the cost calculation of the impacts of climate change, but also of climate change mitigation measures.

Turning to the estimations of the costs of climate change mitigation. It can basically be done in two ways and both of them are complimentary and we’re going to be using both of them.

One is rather simply to look at the resource costs of measures, including the introduction of low common technologies and other changes that are necessary such as changes in land use. And to compare the cost of these measures with the costs of the business-as-usual alternatives. So what would happen if we didn’t invest in climate change mitigation?

Another type of analysis more complex but also very useful is to use macroeconomic models to explore the system-wide effects of the transition to a low common economy. It can be very useful in tracking the dynamic interactions of the different factors over time, including the response of the economies to the changes in relative prices. That can be complex for sure, with their results affected by a whole range of assumptions, but they still provide very useful insights.

On the basis of these two methods, so the bottom-up technological analysis and the more sophisticated macroeconomic analysis, the central estimate is that the stabilization of greenhouse gases emissions at level of 500 to 550 parts per million of C02 equivalent will cost on average around 1% of annual global GDP by 2050. The results of the bottom-up technology modeling range from minus 1%, so even in that gain compared to the business-as-usual, to plus 3.5%. So a net cost of GDP and the results of a top-down macroeconomic modeling range from minus 2%, so again, a net gain to plus 5%, a net cost of GDP. This is significant for sure. This is not a marginal cost, but this has to be looked at as an investment cost and not as a pure loss. And this is fully consistent with continued growth and development. In contrast by the way with uncontrolled business-as-usual climate change, which will eventually pose very significant threat to growth and development.

So let’s try to conclude by summarizing everything we’ve learned in this lecture. The first point that’s going to guide the rest of our journey and further exploration of how we meet the challenge of deep decarbonization is that 2-degrees Celsius is really the upper limit for climate safety, because beyond 2-degrees Celsius of global warming, it is our basic ability to adapt to the likely impact of climate change that is at risk. Second, staying within the 2-degree limit is technically feasible and the costs are relatively modest compared to the size of the global economy, that’s one. But also and very importantly, compared to the cost of the potential impacts from climate change itself. So there is a very good economic case  to be made for climate change mitigation. The conclusion is that the 2-degree limit is a very important tool that we have that must be preserved, but also that must be operationalized in the agreement we’re trying to reach in 2015 in Paris, because only an internationally coordinated, goal oriented, it’s very important, approach to climate change mitigation will allow us to be on track and avoid dangerous climate change. The truth is, unfortunately, that very few countries have looked seriously at the implications for them of staying within the 2-degree limit. So that’s why in the next lectures we will explore in detail, but also country by country the deep transformation of the global economy and in particular, of the energy systems that are required to stay within the 2-degree limit.

The 2-Degree Limit III

Limiting the Mean Surface Temperature Increase Below 2-Degrees Celsius vs. Pre-Industrial Levels

So, we just discussed what would be the absolutely catastrophic consequences of climate change if we did nothing to control the rising greenhouse gases emissions.

Now, I want us to spend some time discussing what we could do to avoid these catastrophic consequences and in particular at what level we should try to limit the increase in temperature.331

So as we have already been discussing in 2010, during the Sixteenth Conference of the Parties of the United Nations Framework Convention on Climate Change. The world’s governments gathered and decided for the first time that they would operationalize the ultimate objective of the UNFCCC which so far remained a bit vague and they decided to set a particular quantitative target of limiting the increase in mean surface temperature below 2 degrees Celsius compared to preindustrial levels. Of course this in recognition of the extreme risks for a safety of the planet but also for a future development opportunity of the temperature increase exceeding two degrees Celsius as we just discussed by looking at some of the potential impacts but what is really interesting and what I want us to spend a little bit of time discussing is that the adoption of this particular target was really the result of of a long and complex process.332

It was very interestingly the result both of a scientific but also of a political process. A scientific process first because each assessment report of the Intergovernmental Panel on Climate Change, the IPCC, provided new insights on three things. I mean the IPCC is divided into 3 main working groups: the first is on the basics of climate science, the second is on the risks of climate change and how we can adapt to them, and the third working group is on the opportunities for climate change mitigation on the economics and the policies of climate change mitigation, so each assessment report of the IPCC provided not only by the way more precise estimates but also revised estimates and in general and we’re gonna go through the history of these different assessment reports in general sending each and every time an even more alarming message to the world than the previous reports.

But I said that it was not only the result of the scientific process but also the political process and it’s very important to understand that the two-degree limit is really the result of the combination of science but also politics because some countries or groups of countries and in particular the European Union or the small islands developing states really pushed hard in the negotiations for the adoption of a particular long-term goal to climate change mitigation.333

So let’s look in detail at the key milestone334s in this scientific but also political process. It all began in a way in 1989 when an advisory group to the United Nations Environment Programme or UNEP released what really became a landmark report. One of the key conclusions of this report was that an increase in the mean surface temperature of 2 degrees Celsius is and I quote “An upper limit beyond which the risks of grave damage to ecosystems and of nonlinear responses are expected to increase rapidly.” So that’s one of the very first mention of the 2 degree limit in an official document commissioned by the United Nations Environment Program. One-year later in 1990 the Second World Climate Conference was a very important step towards adopting a global climate change treaty.

The conference was sponsored by The World Meteorological Organization and also by UNEP, and this time it aimed at making very concrete policy recommendations on the run up to the negotiation of the climate  treaty, and one of these recommendations was that the ultimate objective should be to stabilize greenhouse gases concentration at a level that would prevent dangerous anthropogenic interference with the climate system. It should sound very familiar because as we have discussed several times.

Now two years later in 1992 the United Nations Framework Convention on Climate Change was adopted, and its ultimate objective is directly in line with the recommendation coming from The World Climate Conference. So moving on, in 1995 the IPCC released its Second Assessment Report, and it was also a very important report. It looked at the consequences of the doubling of the greenhouse gases concentrations of the time to a level of 550 parts-per-million of CO2 equivalent, and here one-year later in 1996 enters the European Union which played a very instrumental role in having the two-degree limit adopted as the global goal of climate change mitigation because in 1996 the European Union Environment Council picked up the conclusion from the IPCC Second Assessment Report, and it said the European Union Environment Council that it believed that global average temperature increase should not exceed two degrees Celsius above pre-industrial levels and therefore that the concentration levels of greenhouse gases lower than 550 parts-per-million CO2 equivalent should guide the global greenhouse gases limitation and reduction efforts.335

This is in fact the very first time that the two-degree limit is picked up politically as opposed to only in a scientific report but what is also interesting is to look at the concentration of greenhouse gases that was at that point in time associated with the two  degree limit because again at that point in time we are going to  see that after that, science made progress, but at that point in time it was considered that stabilizing greenhouse gases concentrations at 550 parts-per-million would be sufficient to limit the increase in temperature below 2 degrees Celsius.

After that came the Third Assessment Report of the IPCC in 2001, and it revised and it revised pretty significantly  the analysis included in the Second Assessment Report, and in many ways the conclusions of the Third Assessment Report were more pessimistic and the message that was sent was even more alarming. In particular, the projected increase in temperature over the next century had increased from a range of 1 to 3.5 degrees Celsius in the Second Assessment Report to 1.4 to 5.8 degrees Celsius in the Third Assessment Report.336

Again, the European Union was key in the process and in 2005 it organized a Heads of State Summit this time and not only a meeting of the Environment Ministers. It organized The Heads of State Summit to discuss the issue of climate change and to define the European Union position in the upcoming international negotiations, and it concluded that and I quote, “With a view to achieving the ultimate objective of the UNFCCC, the global average surface temperature increase should not exceed 2 degrees Celsius above pre-industrial levels but what is really interesting and what I want you to focus on is that the European Union Environment Council added that based on the recent scientific research from the IPCC it became unlikely that the stabilization of concentrations above 550 parts-per-million of CO2 equivalent would be consistent with staying within the two-degree limit and that in order to have a reasonable change of limiting global warming to no more than 2 degrees Celsius then the stabilization of greenhouse gases concentrations well below 550 parts-per-million of CO2 equivalent may be needed.337

Two years after that in 2007 the IPCC released another this time The Fourth Assessment Report, and it concluded that in order to limit the temperature increase below 2 degrees Celsius, the concentration of greenhouse gases emissions would have to be stabilized below 450 parts per million instead of the 550 parts-per-million as was previously sought. Following the publication of the IPCC Fourth Assessment Report, the European Union Environment Council concluded that the IPCC Fourth Assessment Report demonstrates that keeping the two-degree objective within reach requires the stabilization of the concentration of greenhouse gases emissions in the atmosphere in line with the lowest stabilization level that was assessed in the report and that was 450 ppm of CO2 equivalent, but interestingly, and I want to emphasize this part compared to the previous assessment reports.

The Fourth Assessment Report did not only revise its analysis of the temperature response of the planet to greenhouse gases concentrations it also revised its analysis of the risks induced by global warming of 2 degrees Celsius and some of the risks that were previously estimated to be only limited became this time assessed as severe or even very severe but in spite of this new assessment of the risks induced by even only two degree of global warming, the political support for the two-degree limit grew and many other countries such as Chile or New Zealand or Norway or South Africa or Switzerland to quote only a few of the much larger number rallied the European Union position to make the two-degree limit the long-term goal of global climate change mitigation  efforts.338

It is very important to stress that not everybody agreed with that particular objective and in particular the small islands developing states were arguing for a 1.5 degree Celsius limit or to put it in concentration equivalent–a 350 parts-per-million concentration of greenhouse gases—because they were pointing and very rightly so to the risks to their very survival of global warming of 2 degrees Celsius because in particular of the rise in sea levels we discussed that would result from that, but even though not everybody agreed to 2 as opposed to 1.5 degrees Celsius, the Copenhagen Accord was adopted in 2009 as a political declaration and then put back into the legal framework of the UN Convention on Climate Change in 2010 and this is really the first time that within the legally-binding instrument internationally there is not only a recognition that the two-degree limit should guide the global mitigation efforts but also the recognition of the need to consider the strengthening of the long-term global goal in particular maybe to 1.5 degrees Celsius.

339So this is the history of the process as you can see; inputs from science but also in the end the very political decision to make the two-degree limit the long-term global goal of mitigation effort. here is to be frank some irony one might put it like that in the fact that as we’ve seen as new scientific evidence became available the temperature increase resulting from a given concentration tended to be higher than previously thought and so tended to be the impacts resulting from a given temperature increase in particular 2 degrees Celsius so in fact 2 degrees Celsius has long been in the thoughts of people as the objective but it somehow changed meaning as scientific evidence progressed because the impacts attached to that particular target were considered to be even more significant than previously thought but in the meantime global greenhouse gases emissions continue to increase and at an always faster rate and the window of opportunity to stay within the two-degree limit closes very rapidly which is a very big problem because as we’ve seen two-degrees is really the upper limit for safety so we’re gonna try to see what we can do to keep that target within reach.

The 2-Degree Limit II

The Consequences of the BAU Trajectory

So we’re going to be discussing the consequences of what I just described, so the consequences of these rising greenhouse gases emissions, of the rising concentration, and of the resulting temperature increase.

The truth is that, in short but we’re going to go in further detail in a minute, that global warming of 4 degrees Celsius or more would have very severe and irreversible impacts and the truth is that these impacts would really threaten the continuation of economic development in the future and also to be frank threaten our ability to adapt to these impacts if global warming were to even exceed 4 degree Celsius so in the previous lectures we’ve already looked at some of the examples in many countries of climate change that is already occurring and and some of these impacts but now we’re gonna look a bit more systematically and also a bit more theoretically at some of the most important risks of climate change going forward and in particular we’re going to discuss three among these most important risks.322

One being the rise in sea levels associated with global warming, the other being the acidification of oceans as the concentration of CO2 increases in the atmosphere since the oceans trap part of this increase in concentration, and the third and again we’ve already looked at evidence of them happening, is the increased frequency and also the increased intensity of extreme weather events such as heat waves and or floods.

So let’s look first at the rise of sea level. How can the increase in CO2 concentration result in a rise of the see. Well, the mechanism is pretty complex but if we try to simplify it, the rise in the level of the sea is caused by the thermal expansion of the oceans but also by the addition of water to the oceans as a result of two things: the melting and the discharge of ice from mountain glaciers but also ice caps and from the much larger Greenland and Antarctic ice sheets.

So let’s try to quantify a little bit the rise in sea levels that would result from global warming. The estimate is that global warming of 4 degrees Celsius would lead to a sea level rise of approximately 0.5 to 1 meters possibly more according to some estimates by the end of the century. But there is something very important you need to understand here and it is that the rise in sea levels is a very slow process that would continue long after the concentration of greenhouse gases emissions has stabilized so if greenhouse gases emissions led to 4 degrees Celsius of global warming, the rise in sea levels would not stop at 0.5 or 1 meter by the end of the century.

It would be several meters higher in the coming centuries adding to the risks induced by global warming. Even if greenhouse gases emissions were reduced in order to limit global warming to only 2 degrees Celsius the sea level rise would be about 0.3 to 0.8 meters by the end of the century but again and let me stress that it wouldn’t stop by the end of the century and it would continue to rise after and most estimates range in between 1.5 and 4 meters of sea level rise by the year 2300 and I should also point out that some estimates and not funny estimates but really really serious estimates even forecast a 6 meter rise in the sea level in the long run associated with a two-degree only of global warming.

The truth is that the sea level rise would likely be limited to below 2 meters in the long run only if global warming was kept to well below 1.5 degree Celsius, maybe something like 1 degree Celsius so why is it a problem It is pretty obvious that such an increase such a rise in the sea level would have really harmful consequences in particular because a very significant fraction of the world population is settled along the coastlines and and often in in large cities so a sea-level rise that would be so gigantic and in the event of climate change and global warming exceeding two degrees Celsius would mean that a very large fraction of  today’s places inhabited by a large fraction of the global population would be underwater so that really makes sea level rise potentially one of the most long-term but very severe impacts of climate change.

Let’s discuss another very serious potential impact from global warming and that is the acidification of the oceans. I’ll explain in a minute what we mean by the acidification of the oceans but let’s look first at the role that the oceans play in the carbon cycle. In fact fact they play a major role because they are one of the Earth’s largest sinks for CO2 emission so as the atmospheric concentration of CO2 rises the oceans absorb the additional CO2 in an attempt to restore the balance in between the uptake and the release of CO2 at the oceans’ surface and this process directly impacts the ocean’s biogeochemistry because the CO2 reacts with water to eventually form a weak acid and this process we call the ocean’s acidification.

So why is that a problem, the acidification of the oceans. Well we only have to look at the past to look at what it would mean in the future and to realize how serious a threat it is because in the past such observed changes in the oceans’ acidity have very often been associated with large-scale extinction events, of extinction of marine species. So the problem is really that these changes in acidity are projected to increase in the future as a result of global warming and an increasing concentration of greenhouse gases emissions in in the atmosphere so that would really add another piece of very significant stress to the marine ecosystems when they’re already under tremendous pressure from human activities such as overfishing and pollution.

The oceans’ acidity already increased by about 30 percent compared to the pre-industrial level but if the temperature increased by 4 degrees Celsius then the oceans’ acidity would increase by another 150 percent and that would really have very severe negative consequences on the oceans’ ecosystems and on the marine species and therefore on food supply because we in part rely on the ocean to prove our food. More specifically it would also have very severe consequences for coral reefs and it might sound like nothing but it clearly isn’t because the coral reefs perform very important environmental services but the truth is starting at perhaps something like 550 parts per million of CO2 equivalent, so much before the temperature increase would reach four degrees Celsius, the coral reefs are expected to start to dissolve.

Let’s move on to the third type of potential impacts I was mentioning and that is the increased frequency but also intensity of some extreme weather events and and let’s discuss at first the narrow issue of rising temperatures and the heat waves resulting from these increases in temperature. The very basic and and very important point to understand is that a global warming of 4 degrees Celsius or more would not be evenly distributed across the world so that’s the world average but some parts of the world would experience a much higher increase than the world average when some other parts of the world would experience a more moderate increase but with global warming of 4 degrees Celsius on average it means that increases of six degrees Celsius or more average monthly summer temperatures would be expected in in very large regions of the world including in particular the Mediterranean, North Africa, also the Middle East but also some parts of North America so it means that recent extreme heat waves such as own those experienced by Australia, California, or Russia recently are likely to become the new normal in a 4 degrees Celsius world and tropical South America and Central Africa all the tropical islands in the pacific are likely to regularly experience summer heat waves of really unprecedented magnitude but also durations and in regions such as the Mediterranean North Africa and the Middle East but also the Tibetan Plateau almost all summer mnths are likely to be warmer than the most extreme heat waves presently experienced.

To pick only one example which I think reveals a lot: the warmest July in the Mediterranean region could be 9 degrees Celsius warmer than today’s warmest July. Do you realize what 9 degrees warmer would mean for a region like the Mediterranean and in summer for the ability, for example, to grow crops?

So the extreme heat waves in the recent years have already had very severe impacts, they have caused heat-related deaths, they’ve caused forest fires such as those we’ve seen recently in Russia or Australia, they’ve also caused very significant harvest losses as we’ve seen recently in California and the truth is that the impacts of extreme heat waves projected for a 4 degree warmer world have not yet been thoroughly evaluated by the scientists but evidence shows that they could well exceed our capacities and the the capacities of some natural systems simply to adapt to the same changes putting us into very severe risks.

Global warming of 4 degrees Celsius would also increase the frequency of heavy precipitation. This is another potential impact from global warming that would not necessarily be felt again evenly across the globe but could have very severe consequences on some parts of the planet and this is especially the case in the high latitudes and in the tropical regions in particular through tropical cyclones but also in winter in the northern mid-latitude.

Interestingly even in some regions where the total precipitation would decrease as a result of global warming, heavy precipitation could increase and that would result pretty ironically in the combined risks of drought and floods in these regions and these extreme weather events generally could induce the breakdown of infrastructure networks and really critical services such as our power production infrastructure or our water supply networks or our health services and that would really have very severe human and economic costs so to summarize the environmental but also I want to stress the economic and social risks of uncontrolled climate change are absolutely immense.

They really threaten to rollback the fruits of decades of growth and development, to undermine prosperity, to jeopardize countries’ ability to achieve even the most basic social economic goals in the future including the eradication of poverty and some in the developed world may think that we could be immune from such risk but what we just saw really proves that this is not true them.

Unabated climate change leading to global warming of 4 degrees Celsius or more would really affect all the developed and the developing countries maybe not exactly alike put the effect of such global warming would be felt in every part of the globe and the truth is that it’s our basic ability to adapt to the effects of this climate change that is at risk if global warming exceeds 4 degrees Celsius in the coming centuries

The 2-Degree Limit I

The Business As Usual Trajectory

Today we’re going to be discussing more specifically about a particular target that we have already mentioned, the objective of limiting the temperature increase to less than 2 degrees Celsius compared to the pre-industrial level, the target that was adopted by the world government in 2010 in Cancun.

We have already discussed in the previous lectures the basic elements of the climate science. We have also shown what are the key milestones on the road to Paris in 2015 when an ambitious agreement on climate change need to be reached but I’m want to start this lecture by describing what are some of the possible consequences of climate change if we don’t control the greenhouse gases emissions that we need to.

I want to be discussing where the business as usual trajectory leads us so if we continue with the current trends of rising emissions because we’ve seen already some of the impacts of climate change that are already occurring because climate change is real and is already happening in many parts of the world but if greenhouse gases concentrations continue to rise the way they’re currently doing these impacts would be really getting more and more severe overtime so let’s start this first chapter of lecture 3 by discussing where the business as usual trajectory, the continuation of the current trends, would lead us and what would be the consequences.


As we’ve said several times now the United Nations Framework Convention on Climate Change or the UNFCCC was adopted in 1992 and entered into force two years later in 1994 and by now you should all almost know by heart what is the ultimate objective of the UNFCCC, that is stated in its Article 2, which is to stabilize greenhouse gases concentrations at a level that prevents dangerous anthropogenic interference with the climate system in order to enable economic development to proceed in a sustainable manner.


The last part of the sentence is very important because it shows very well that we’re not aiming only for an environmental objective were not only aiming to protect the planet we’re aiming to protect planet in order to let economic development proceed in the future so this commitment to  stabilize greenhouse gases concentrations was re-iterated at basically each and every conference of the parties or COP of the UNFCCC since 1992 and in 2010 as we’re going to discuss in more detail today, the parties to the UNFCCC even adopted a quantified target for the first time to operationalize the objective of the UNFCCC, as I said, they agreed to limit the rise in mean surface temperature below two degrees Celsius compared to pre-industrial level and in this lecture we’re going to explain why they chose this particular target and why it is so important for sustainable development going forward but the truth is that in spite of these repeated commitments each and every year to stabilize greenhouse gases concentrations, also in spite of the growing awareness of the potentially catastrophic impacts of uncontrolled climate change and in spite of a growing number of climate change mitigation policies the anthropogenic greenhouse gases emissions, the greenhouse gases emissions that are human-induced are fall from stabilizing as opposed to the goal the UNFCCC quite the country in fact, they continue to increase, continue to increase steadily, and they even continue to increase very rapidly.312

In fact the average annual growth rate of anthropogenic greenhouse gases emissions has been even higher during the last ten years than during the previous thirty years. From the year 2000 to the year 2010 we have added approximately 1 billion tonnes or one gigatonnes as we’re going to say of CO2 equivalent for year on average in the atmosphere so remember we use the unit of CO2 equivalent to find a common metric to add together all of the different greenhouse gases of which carbon dioxide CO2 is only one. So this rise, this addition of one billion ton of Co2 in the atmosphere per year from the year 2000 to the year 2010 represents on average a 2.2 annual rate increase and so my point is that this is even more than the 0.4 billion tonnes of CO2 equivalent per year on average that we added in the atmosphere in the previous thirty years from the year 1972 to the year 2000 which represented of course an increase but an increase of only 1.3 percent per year as opposed to the 2.2 percent of the last ten years and so as a consequence of this continued and accelerated in fact increase, the anthropogenic greenhouse gas emissions reached their highest level in  human history in 2010 which is the latest data that we have available so to put a number on these emissions and one you need to remember because it’s going to be important going forward in 2010 the annual anthropogenic greenhouse gases emissions stood at 49 billion tonnes of CO2 equivalent that’s the level of the emissions per year today or in 2010 and this is what you show on this graph and you show the increase in the total greenhouse gases emissions from the year 1970 to today to 2010 and you show that divided by the main types of greenhouse gases so in particular CO2 coming both from the burning of fossil fuels and from industrial processes, the CO2 coming from a different source of human activities, the deforestation and agriculture and also methane emissions and nitrous oxide emissions.

313So the bottom line looking at this graph is that since 1970 all major sources of greenhouse gases emissions have increased. As I said, the CO2 emissions but also all the other greenhouse guess but what I want you look at in particular is to the increase in CO2 emissions from the burning of fossil fuels and from other industrial processes because this rise in particular was spectacular and is going to be the main focus of this course. It contributed to about 78 percent of the total greenhouse gases emissions increase since 1970 so today the fossil fuel related CO2 emissions are the single largest source of  greenhouse gases emissions in 2010, it reached 32 billion homes per year and represented approximately 65 percent of the total greenhouse gases emissions and since 2010 these emissions still grew further by about 3 percent between 2010 and 2011 and again by something in between 1 and 2 percent in between 2011 and 2012 and so where did these emissions come from? Which are the sectors that are responsible for these rising CO2 emissions?

Well first they came from the energy supply directly for 47 percent, then they came from the industry sector for about 30 percent. After that from the transport sector for 11 percent and the remaining CO2 energy emissions came from the building sector for 3 percent. So what are the drivers of these rapidly-growing greenhouse gases emissions? Well since 1970 economic growth and population growth were the most important drivers of this increase in emissions.


The contribution of the population growth to the increase in emissions during the past 10 years remain roughly the same compared to the previous three decades but the contribution to economic growth to the increase in emissions has risen very sharply in the past 10 years compared to the previous three decades and as we already pointed out in some of the previews lectures this is in part the result of good news and that is the very strong catch-up economic growth in the middle-income countries and in particular in China but of course that came with side effects and in particular rising levels of greenhouse gases emissions so these are the two most important factors that explain the rise in emissions in the recent past: the population rose and the economy grows.

On the other side since 1970 the energy efficiency of economic growth has pretty significantly improved. It means that progressively we were able to manage to consume less energy per unit of GDP which is the good news in a way from the climate change mitigation perspective because it means that we were also adding less unit of greenhouse gas emissions per unit of GDP produced but the bad news on the other side is that this improvement of energy efficiency was too small compared to the other driving force going in the other direction:the population and the economic growth, in particular the increase in income per capita.315

Since 1970 the carbon intensity of energy also decreased. What does that mean? Well it means, progressively we were emitting less CO2 per unit of energy consumed which again was the good news from the climate change mitigation perspective but here too this improvement in the carbon intensity of energy was much too small compared to be the necessary emission reductions to avoid dangerous climate change but what I want to point out here and what is in a fact very worrying is that during the past ten years the increased use of fossil fuels but in particular of coal relative to the other energy sources has reversed the long-standing trend of gradual decarbonization of the world energy supply.

It means that the carbon content of energy is increasing during the past ten years we emitted more carbon emissions per unit of energy we consume when instead this trend should be decreasing and it should be decreasing very strongly and very quickly. So the current trends are clearly going in the wrong direction but the question is where do they lead us? What would be the global warming consequence of these rising emissions going forward so let’s do the math.317

I mean let’s look at where we are today and and let’s look at where we would be if the current trends were continued in the coming years so where are we today? In 2011, the CO2 equivalent concentration was estimated to be at approximately 430 parts per million remember parts per million is the unit we use to measure the concentration of greenhouse gases in the atmosphere.

If the current trends continued then the concentration of these greenhouse gases in the atmosphere would exceed 450 parts per million of CO2 equivalent as soon as 2030. I am stressing this particular number of 450 parts per million because we’re going to see when we discuss two-degree limit that this is the concentration of greenhouse gases emissions that corresponds to the two-degree limit in the temperature increase but it wouldn’t stop at 450 parts per million in 2030; it would be somewhere in between 750 and perhaps even more than 1300 parts per million of CO2 equivalent by the end of the century so what would be the consequence of that amount of greenhouse gases emissions in the atmosphere?

Well that would be very dangerous because it would result in global mean surface temperature increase by the end of the century from somewhere in between 4 and 5 degrees Celsius compared to  pre-industrial level and the truth is that if you build into the analysis all the uncertainty regarding the reaction of the climate to the greenhouse gas emissions then this range could expand potentially up to 8 degrees Celsius of warming before the end of the century and the consequences of such an increase in temperature would be really catastrophic.