Chapter 3: Projected climate change and its impacts

Chapter 3 deals with projected climate change and its impacts. This is probably the most interesting for the general public because everybody wants to know what will happen in the future.

It begins by saying that global GHG emissions will continue to increase even with the world's attempt at sustainable development. The IPCC Special Report on Emission Scenarios (SRES, 2000) predicts that emissions from fossil fuels will be most responsible for an increase in GHGs of around 25% - 90% as fossil fuels will be the dominant source of energy until 2030. It is predicted that this will lead to a 0.2 degree Celsius increase per decade, so the climate will essentially rise by another 1 degree within the next 50 years, which will take us to the 'safe limit' of 2 degrees warming (however, 2 degrees warming will severely impact Africa and small island nations). Here it points out an important positive feedback mechanism; that as the oceans warm, they become less capable of storing CO2 and therefore the concentration of atmospheric CO2 will increase, inducing further warming. Even if GHG emissions were stabilized at 2000 levels, an increase in temperature of 0.1 degrees Celsius per decade is expected.

In order to cover the range of possible future impacts of climate change, the IPCC has come up with a number of emission scenarios and the expected temperature increase relative to 1980 - 1999. Ultimately, the scenario our world takes will be determined by the actions of industry, politicians and people alike. The main scenarios are shown below:

• year 2000 constant: temperature increase of 0.3 degrees above 1980 - 1999
  
• B1 - GHGs increase from 40GTCO2 eq/yr to a peak of 55GTCO2 eq/yr in 2040 and then drop to 25GTCO2 eq/yr by 2100. This is the most optimistic scenario and is expected to cause an increase of 1.9 degrees by 2100 (above 1980-1999 average)
• A1B - GHGs increase from 40GTCO2 eq/yr to a peak of 75GTCO2 eq/yr in 2050 and then decrease to 60GTCO2 eq/yr by 2100. This leads to a temperature increase of 2.9 degrees by 2100.
  
• A2 - this is the scenario I believe is most likely, judging by the lack of will of Chinese and American political leaders and lack of consensus between the global north and south, and the general lack of a sense of urgency shown at Copenhagen COP15. Here, GHG emissions continue increase linearly reaching a peak in 2100 of almost 140GTCO2 eq/yr. Best estimates say this would lead to a temperature increase of around 3.4 degrees. Very dangerous situation!

Some systems and sectors will be particularly affected:

• ecosystems: in particular, tundra, boreal forest and mountain regions due to sensitivity to warming; Mediterranean ecosystems due to reduced rainfall; tropical rainforests where rainfall is expected to decrease; coastal mangroves and salt marshes due to multiple stresses (increased temp, sea level, changing wind and rainfall patterns); marine coral reefs due to multiple stresses
• water resources in dry regions at mid-latitudes and in the dry tropics
• agricultural output in low latitudes due to reduced rainfall
• low lying coastal areas due to increase sea level and increased threat from extreme weather events
• human health in poor countries

Regions that will be particularly affected:

• the arctic, because of high rates of warming (the arctic is expected to experience the highest temp increase)
• Africa, because it is poor and has a low adaptive capacity. Africa is expected to experience greater warming than many other parts of the globe
• small islands where there is high populations e.g. Maldives
• Asian and African mega-deltas, due to high population densities, low adaptive capacity, risk of storm surges and river floodings

Richer regions have a higher adaptive capacity, so will be less affected in human terms, but some segments of society (the poor, the elderly) will be.

Ocean Acidification

The ocean is becoming increasingly acidic due to absorption from the atmosphere which will have a negative impact on marine shell forming organisms, such as corals, which will impact those other organisms that depend on them. In short, no coral reefs would mean the oceans would be almost void of life.

Sea level rise

It is important to note that the scenarios for predicted sea level rise do not include the real possibility of rapid melt as temperatures continue to increase. This is a little unrealistic, but I guess the IPCC find it too difficult to calculate. So, the figure given here are very optimistic and for that reason I shown the upper ranges for each scenario:

• B1 - 1.8 degrees = 0.38 metres
• A1B - 2.8 degrees = 0.48 metres
• A2 - 3.4 degrees = 0.51 metres

The report also includes a summary table (table SPM2) that lists the possible regional impacts by continent. Table SPM3 lists the possible impacts on certain sectors, which includes both natural and human systems. These are definitely worth a look.

Warming could lead to some changes that are abrupt and/or irreversible. Here 'abrupt' means the melting of ice sheets on polar land, which is expected to happen over millennial time scales, but which may occur over centuries i.e. 'abrupt'. Irreversible impacts would include the loss of biodiversity, as it is expected that a 2.5 degree increase would wipe out 30% of species and a 3.5 degree increase is expected to wipe out 40 - 70% of all species.

Atlantic Ocean Circulation is expected to slow down over this century, but not stop completely. This means that Europe will continue to warm. The changes in the ocean circulation is expected to have corresponding impacts on marine ecosystems.

This chapter finishes by pointing out that due to the timescales on which the climate system operates, even if GHG emissions are stabilized, anthropogenic warming and sea level rise would continue for centuries. Though not specifically mentioned, I assume this is due to atmospheric residence times of GHGs.