For those not familiar with it, the Intergovernmental Panel on Climate Change was set up in 1988 to write periodic assessments of the state of climate science. Its goal is to produce policy-neutral reports that inform policymakers about the best thinking of the scientific community. These reports have tremendous impact on the debate, owing to the credibility of the IPCC process.

The IPCC is actually split into three working groups. Working group 1 focuses on basic climate science, working group 2 focuses on the impacts of climate change and human adaptation to it, and working group 3 focuses on mitigation efforts (efforts to reduce carbon dioxide emissions). In 2007, as part of the IPCC’s fourth assessment report, each of the three working groups issued a report (e.g., see here for a discussion of the working group 1 report).

Now comes the final part of the fourth assessment report: the synthesis report. This report ties together the three working group reports in an effort to create a single unified picture of what we know about climate change.

Yesterday, the IPCC released the Summary for Policymakers of the synthesis report. Given the increasing political and social importance of climate change, and the IPCC’s recent Nobel Prize, it is no surprise that the release of this report is front page news across the planet.

So what’s in this new report? To begin with, it is a summary of the previous working group reports that came out in 2007. So one would not expect anything really new here. That said, there are several places where the IPCC took the opportunity to clarify and amplify positions that my not have come out with sufficient clarity in the previous reports.

The first part of this new summary covers the working group 1 material, and reiterates the main conclusions of that report:

  1. The warming of the climate system is unequivocal.
  2. Humans are very likely responsible for most of the recent warming.
  3. Without intervention, the Earth will likely experience a warming of a few degrees Celsius over the 21st century.
  4. This warming will lead to a host of other changes, such as: changing precipitation patterns, sealevel rise, and melting glaciers, increased chances of drought, etc.

One of the most misrepresented aspects of the working group 1 report was its estimates of sea level rise. The 2007 report contained a slightly lower estimate for 21st century sea level rise than the 2001 report. The denial-o-sphere jumped on that as evidence that the impacts of climate change were being revised downward — so don’t worry your pretty little head about it. That argument was a misrepresentation of report, and the IPCC used the synthesis report to make a strong statement about it:

Because understanding of some important effects driving sea level rise is too limited, this report does not assess the likelihood, nor provide a best estimate or an upper bound for sea level rise. Table SPM.1 shows model-based projections of global average sea level rise for 2090-2099. The projections do not include uncertainties in climate-carbon cycle feedbacks nor the full effects of changes in ice sheet flow, therefore the upper values of the ranges are not to be considered upper bounds for sea level rise. They include a contribution from increased Greenland and Antarctic ice flow at the rates observed for 1993-2003, but this could increase or decrease in the future.


The next part of the report covers the impacts of climate change, based on the results of the working group 2 report. The bottom line is that the scientific community expects serious and important regional impacts due to climate change. These are broken down in reasonable detail in the report, so I won’t repeat them here. However, there are a few general conclusions that merit repeating:

  1. Altered frequencies and intensities of extreme weather (heat waves, heavy precipitation events, droughts, more severe storms, extreme sea level events), together with sea level rise, are expected to have mostly adverse effects on natural and human systems.
  2. Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.
  3. Anthropogenic warming and sea level rise will continue for centuries due to the timescales associated with the climate processes and feedbacks, even if GHG concentrations were to be stabilized.
  4. Climate change preferentially selects the poorest and most vulnerable in any society.

The last and most interesting part of the report discusses what we can do about climate change. In this section, the IPCC makes strong positive statements that we can do something about the problem:

Both bottom-up and top-down studies indicate that there is high agreement and much evidence of substantial economic potential for the mitigation of global GHG emissions over the coming decades that could offset the projected growth of global emissions or reduce emissions below current levels.

An effective carbon-price signal could realise significant mitigation potential in all sectors. Modelling studies show global carbon prices rising to 20-80 US$/tCO2-eq by 2030 are consistent with stabilisation at around 550 ppm CO2-eq by 2100. For the same stabilisation level, induced technological change may lower these price ranges to 5-65 US$/tCO2-eq in 2030.

And, oh yeah, mitigation efforts actually make economic sense:

There is high agreement and much evidence that mitigation actions can result in near-term co-benefits (e.g. improved health due to reduced air pollution) that may offset a substantial fraction of mitigation costs.

And (as most Grist readers know) we can reduce our emissions with existing technology:

There is high agreement and much evidence that all stabilisation levels assessed can be achieved by deployment of a portfolio of technologies that are either currently available or expected to be commercialised in coming decades, assuming appropriate and effective incentives are in place for their development, acquisition, deployment and diffusion and addressing related barriers.

Overall, this report is required reading for anyone seeking to be literate in the climate change debate. Download it and read it today!