The key message of the concluding communication from the Copenhagen Climate Science Congress, attended by some 2000 scientists and others, 11-12 March, 09, reads:

Recent observations confirm that, given high rates of observed emissions, the worst-case IPCC scenario trajectories (or even worse) are being realized. For many key parameters, the climate system is already moving beyond the patterns of natural variability within which our society and economy have developed and thrived. These parameters include global mean surface temperature, sea-level rise, ocean and ice sheet dynamics, ocean acidification, and extreme climatic events. There is a significant risk that many of the trends will accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts.

The worst case scenario of the IPCC-2007 (AR4) is defined as “scenario A1F1”, which assumes global CO2-equivalent emissions (CO2-e = CO2 + methane + nitric oxide) will grow from the current level of about 40 billion ton CO2-e per year (GtCO2-e/year) to near 130 GtCO2-e/year through the 21st century.

The consequences of this IPCC Special Report on Emission Scenario (SRES) is a mean global temperature rise of between about 2.4 and 6.5 degrees Celsius by 2090-2099 relative to the period 1980-1999, taking the albedo effects of atmospheric aerosols (industrial haze, dust, carbon particles) into account. However, the IPCC-2007 AR4 Report takes only limited account of carbon cycle feedback effects and ice/water interaction feedback effects in raising global temperatures. It also acknowledges limited information regarding ice sheet melt and breakdown dynamics.

The last time mean global temperatures reached 2 to 3 degrees Celsius above present levels — in the mid-Pliocene (3 million years ago), an event associated with CO2 levels of about 400 parts per million — polar regions were heated by near-8 degrees C and sea levels rose by 25+/-12 meters relative to the present. This represents near-total melting of Greenland and west Antarctica ice sheets.

A rise of mean global temperatures above 4 or 5 degrees Celsius would shift the atmosphere to pre-glacial/interglacial conditions, which dominated the Earth from about 34 million years ago (end-Eocene).

Key message No. 5 of the Copenhagen Congress reads:

There is no excuse for inaction. We already have many tools and approaches: economic, technological, behavioral, management to deal effectively with the climate change challenge. But they must be vigorously and widely implemented to achieve the societal transformation required to decarbonise economies. A wide range of benefits will flow from a concerted effort to alter our energy economy now, including sustainable energy job growth, reductions in the health and economic costs of climate change, and the restoration of ecosystems and revitalization of ecosystem services.

Whereas at Copenhagen hopes the December meeting of world governments in the same city will make the difference are limited, the role of climate scientists in explaining the implications of non-decision is critical. According to Senator Christine Milne “Australia’s climate scientists have been remarkably reticent to publicly criticise what they have in private slammed as a totally unacceptable and inadequate target”.