IPCC AR5 – WG1 Physical Science Basis

The UN Intergovernmental Panel on Climate Change (IPCC) now says we are as certain that humans are dramatically changing the planet’s climate as we are that smoking causes cancer.” (J. Romm)

Working Group 1 of the IPCC (Intergovernmental Panel on Climate Change) has issued the first part of Assessment Report 5. This Report covers the Physical Science Basis for anthropogenic climate change.

Global warming is confirmed yet again: “Total radiative forcing is positive, and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.” This statement is backed up by direct measurements of the climate system.

And it is human caused: “Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.” …and… “It is extremely likely that human influence has been the dominant cause of observed warming since the mid-20th century.

Even models are better: “Climate models have improved since AR4. Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions (very high confidence).

Dealing with projections and required action: “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.

There is little change in the climate sensitivity (still in range 1.5 to 6). The “transient” climate sensitivity (that likely over decades or centuries) is given as in the range 1 to 2.5 (sensitivity is the expected rise in temperature for a doubling of CO2). The response to the cumulative quantity of GHG emissions

These statements are based on a broad range of measured data from across the entire climate system and on close monitoring of our burning of fossil fuels and other human activities such as forest clearing.

Of course, to most this is no surprise. The main message of the report is that since AR4, nothing much has changed only the urgency to act has increased. It is all the more certain that we are heading for disaster if we don’t cut emissions. Scientists hold the same confidence in the evidence that smoking kills as they do that humans are causing global warming.

The huge effort on research in the last 7 years has failed to find any doubt that we are causing our planet to warm by our use of fossil fuels and that we must act swiftly to stop this or suffer serious and long term consequences.

So, the science is settled (and has been for some time). What then should we do….

The Report uses projections of how the climate will change. These are based on new scenarios that relate to the amount of GHGs in the atmosphere (Representative Concentration Pathways: RCP2.6, RCP4.5, RCP 6.0 and RCP8.5, Skeptical Science has a good rundown on these… http://www.skepticalscience.com/rcp-guide-part1-post.html).

These pathways are similar to previous future scenarios in that they give a range of human emissions from strong reductions in emissions (RCP2.6) to business as usual emissions (RCP8.5). RCP8.5 sees global temperature approaching 4.5 degrees by 2100 – a disaster scenario. The modeling is conservative because it omits some positive feedback effects such as permafrost melt and loss of arctic sea ice.

There is little difference to the previous AR4 in that strong cuts are required to give us any chance of staying below the 2 degree limit. 2 degrees of rise is the internationally agreed safety rail beyond which dangerous climate change would occur.

Many scientists believe we may be already too late to stay below the 2 degree limit, essentially because of poor decisions made by our leaders over the last few years. Certainly, the failure at Copenhagen had a lot to do with it.

For a 2 in 3 chance of staying below the 2 degree limit, total emissions needs to be limited to a total budget of around 800 GtC – we have so far used up 530 GtC of this amount so there is only 270 GtC still available. This equates to the reduction scenario represented by RCP2.6. Including reduction in aerosols (particles from fossil fuel burning) and melting of permafrost would reduce the emissions budget limit still further.

To achieve RCP2.6, global emissions need to peak before 2020 and reduce at around 5% per year. Current thinking is that RCP2.6 is not a practically achievable scenario as we have already gone past the window to achieve it.

Finally, it must be remembered that the IPCC reports are highly conservative and are regularly revised ‘upwards’. This results from the highly scrutinized process and editing by the many countries that are involved. Consequently, the outcomes of the Report are highly robust and generally we can expect that the reality will turn out to be worse.

Further info…  http://www.realclimate.org/index.php/archives/2013/09/the-new-ipcc-climate-report/    http://thinkprogress.org/climate/2013/09/27/2681861/15-things-ipcc-report/   http://www.businessspectator.com.au/news/2013/9/27/science-environment/warming-hit-home-australians-ipcc .

A few other points to note in AR5:

Atlantic Meridional Overturning Circulation (AMOC) research shows this current is likely to slow (between 10% and 35%). This slowing is likely to impact on the European climate (make the region colder).

Sea Level Rise is expected to be between 0.3m and 1.0m for the range of scenarios (slightly increased from AR4). There is insufficient confidence in the empirical assessments to include them even though they predict up to double the rise. Also, the contribution of Antarctica is still not fully included due to the uncertainty of the actual melting there. Many researchers consider destabilization of marine based Antarctic ice (such as in West Antarctica) to be a serious risk.

Ocean acidification is very well defined as it depends directly on the emissions that occur. The values for reduction in ph (i.e. acidification) are:-   0.15 for RCP4.5, 0.2 for RCP6 and 0.31 for RCP8.5.

Large climatic changes are already locked in and likely to continue for centuries, unless GHG’s are actively removed from the system. Around 25% of the CO2 released will remain in the atmosphere for 1000 years, even if we stopped now.

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