The Copenhagen diagnosis

January 20, 2010

The big climate conference in Copenhagen last December has resulted in a disappointment. Although it can be argued that US president Obama is working very hard to address the climate issue and is making progress, Copenhagen came at a bad moment for the US administration.

Nevertheless, the stakes remain incredibly high. This becomes clear when one reads the Copenhagen Diagnosis, an update on the state of the science of global warming that has been issued by leading climate scientists, independently from the IPCC assessments that are issued every four years. Although the report has been made available already last December (obviously to coincide with the Copenhagen conference), I think it is important enough to call attention to even if I am a bit late to the party. It is freely available from www.copenhagendiagnosis.org. For the impatient, I have copied the Executive Summary below.

The most significant recent climate change findings are:

Surging greenhouse gas emissions: Global carbon dioxide emissions from fossil fuels in 2008 were 40% higher than those in 1990. Even if global emission rates are stabilized at present-day levels, just 20 more years of emissions would give a 25% probability that warming exceeds 2°C, even with zero emissions after 2030. Every year of delayed action increases the chances of exceeding 2°C warming.

Recent global temperatures demonstrate human-induced warming: Over the past 25 years temperatures have  increased at a rate of 0.19°C per decade, in very good agreement with predictions based on greenhouse gas increases. Even over the past ten years, despite a decrease in solar forcing, the trend continues to be one of warming. Natural, short-term fluctuations are occurring as usual, but there have been no significant changes in the underlying warming trend.

Acceleration of melting of ice-sheets, glaciers and ice-caps: A wide array of satellite and ice measurements now demonstrate beyond doubt that both the Greenland and Antarctic ice-sheets are losing mass at an increasing rate. Melting of glaciers and ice-caps in other parts of the world has also accelerated since 1990.

Rapid Arctic sea-ice decline: Summer-time melting of Arctic sea-ice has accelerated far beyond the expectations of climate models. The area of summertime sea-ice melt during 2007-2009 was about 40% less than the average prediction from IPCC AR4 climate models.

Current sea-level rise underestimated: Satellites show recent global average sea-level rise (3.4 mm/yr over the past 15 years) to be ~80% above past IPCC predictions. This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps, and the Greenland and West-Antarctic ice-sheets.

Sea-level predictions revised: By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4; for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as ~ 2 meters sea level rise by 2100. Sea level will continue to rise for centuries after global temperatures have been stabilized, and several meters of sea level rise must be expected over the next few centuries.

Delay in action risks irreversible damage: Several vulnerable elements in the climate system (e.g. continental ice-sheets, Amazon rainforest, West African monsoon and others) could be pushed towards abrupt or irreversible change if warming continues in a business-as-usual way throughout this century. The risk of transgressing critical thresholds (“tipping points”) increases strongly with ongoing climate change. Thus waiting for higher levels of scientific certainty could mean that some tipping points will be crossed before they are recognized.

The turning point must come soon: If global warming is to be limited to a maximum of 2 °C above pre-industrial values, global emissions need to peak between 2015 and 2020 and then decline rapidly. To stabilize climate, a decarbonized global society –with near-zero emissions of CO2 and other long-lived greenhouse gases – needs to be reached well within this century. More specifically, the average annual per-capita emissions will have to shrink to well under 1 metric ton CO2 by 2050. This is 80-95% below the per-capita emissions in developed nations in 2000.

I highly recommend reading the report itself. It is only 64 pages, including clear graphs and a number of gorgeous pictures (and lots of citations of scientific papers for further reading, for those who share my luck of having internet access to peer-reviewed journals). It also very specifically addresses and debunks a number of layman arguments that have often been put forth against climate change (“CO2 increase historically trails temperature increase and therefore it still does now”, etc).

In my opinion the report is an example of scientific outreach at its best. The climate debate has become heavily politicized (the public debate, that is. Although there exists a fierce and lively scientific debate about the details, the genuinely scientific debate on the reality of man-made global warming is long over. The basics of man-made global warming have been firmly established using the same scientific principles that are found in other natural sciences, like my own field of astrophysics). Whatever climate scientists do, it will be held against them: if they don’t invest time and energy in outreach they will be painted as stuck in their ivory towers and out of touch, and if they do, they will be labelled activists. The Copenhagen diagnosis presents a very strong case that outreach from active climate scientists is crucial to the public conversation. And someone who tells a disturbing truth is not by definition an activist.

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