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.

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.

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.

So let’s look in detail at the key milestones 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.

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.

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.

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.

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.

So 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.

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