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.

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

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