The synthesis report from the Copenhagen climate congress of scientists held in March has been released. Its purpose is to offer updates on the science prior to the coming UN conference in December, also at Copenhagen, when new international agreements to reduce greenhouse gas emissions will be sought.
The IPCC 2007 report is the basic document on which policy makers depend, but climate science and climate change are moving quickly; the congress report aims to communicate new knowledge that has emerged that helps our understanding of the impacts of human influence on the climate and the response options available to tackle the issue.
On climatic trends the report is blunt. Greenhouse gas emissions and many aspects of the climate are already changing near the upper boundary of the IPCC range of projections. In the case of sea level rise the rate is even greater than indicated by the IPCC projections. Ocean warming is about 50% greater than previously reported and conservative projections now suggest a sea level rise of around a meter or more by 2100. It will not stop there. Changes in ocean heat content will continue to affect sea-level rise for several centuries at least. Melting and dynamic ice loss in Antarctica and Greenland will also continue for centuries into the future. In fact, global average surface temperature will hardly drop in the first thousand years after greenhouse gas emissions are cut to zero
The unexpectedly dramatic reduction in the area of Arctic sea ice cover is important for climate on a large scale because of the feedback effect of the lowering of reflection of the sun’s radiation. Increased water vapor following warmer global temperature is a further amplifying feedback.
Atmospheric CO2 as well as methane and nitrous oxide concentrations have increased dramatically over recent decades because of human activities. Ice core and sediment records show that the concentration of all of these gases in the atmosphere is now higher than it has been since long before modern humans evolved. In fact, the CO2 concentration in the atmosphere has not been substantially higher than it is now for at least the last 20 million years of the Earth’s history.
The report draws attention to a further amplifying feedback relating to the land and ocean sinks which have so far removed over half the CO2 released into the atmosphere through human activities. A weakening of the sinks is already occurring and there is likelihood that the decrease in the fraction of CO2 removed from the atmosphere by sinks will become larger under high future emissions scenarios. Several effects contribute to this likelihood, including increasing ocean acidification, ocean circulation changes, and water, temperature, and nutrient constraints on land CO2 uptake. Also, previously inert carbon pools can be mobilized and released into the atmosphere either as CO2 or methane, a more potent greenhouse gas. Two areas of concern are tropical peatland carbon, which is vulnerable to land clearing and drainage, and the large stores of organic carbon in Arctic permafrost, which are vulnerable to warming.
Increasing ocean acidity is serious. The report acknowledges that the precise effects are not yet clear, but it is thought that when atmospheric CO2 reaches 450 ppm, large areas of the polar oceans will likely have become corrosive to shells of key marine calcifiers, an effect that will be strongest in the Arctic. The rate of change in ocean chemistry is very high faster than previous ocean acidification-driven extinctions in Earth’s history, from which it took hundreds of thousands of years for marine ecosystems to recover. Ocean acidification will continue to track future CO2 emissions to the atmosphere, so urgent and substantial emission reductions are the only way of reducing the impact of ocean acidification.
On biodiversity generally the report is emphatic. There is a looming biodiversity catastrophe if global mean temperature rises above 2 degrees centigrade, ocean acidification spreads and sea-level rise accelerates. These climate-related stressors will interact with a wide range of existing stressors on biodiversity. The result will be the extinction of a significant fraction of biological species within the next 100 years, a substantially reduced range and higher risk of eventual extinction for other species, and the degradation of ecosystem services on which human well-being depends. Limiting temperature rise to 2 degrees or less, along with strong pro-active adaptation in conservation policy and management can limit the magnitude of the crisis but not entirely eliminate it.
There is much more to the report, but these selections will serve to indicate something of its forthrightness on the science. At 39 pages it is not unduly long, and it has been written to be fully accessible to a wide audience. No negotiator can go to Copenhagen in December claiming that the scientific picture is confusing or less than serious. The report also considers the mitigation and adaptation measures available to avoid the worst effects of dangerous change, and no negotiator can claim that there is little that can be done. The only further ingredient needed is political will.
The report was produced by a writing team of twelve, and has been extensively critically reviewed “to ensure that the messages contained in the report are solidly and accurately based on the new research produced since the last IPCC Report, and that they faithfully reflect the most recent work of the international climate change research community.” This statement from a post on Real Climate indicates that the writers have achieved their aim: “The Synthesis Report is the most important update of climate science since the 2007 IPCC report.”
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