By the time that the IPCC summary of the climate science working group’s fourth assessment report (AR4 – here) was finally delivered to the public last Friday, much of the salient detail had already been leaked to the media.
From a political perspective, among the most important conclusions was the much increased confidence with which the IPCC authors were able to conclude that observed changes are being driven by human activity (>90%, rather than the 66% reported in the third assessment report [TAR]).
Many people commented (as did I) last week that the IPCC review process inevitably produces relatively conservative conclusions, both in terms of the final wording and the scientific basis of the report. This is a good thing – the final document reflects a scientific consensus written in language that makes sense to the decision makers for whom it is intended.
But perhaps as concerning as the conclusions of AR4 are the uncertainties that remain. For example, the report states: ‘Models … do not include uncertainties in climate-carbon cycle feedbacks, nor … the full effects of changes in ice sheet flow…’
Feedback processes are essentially those with outcomes that either amplify (positive feedback) or diminish (negative) the initial perturbation. With respect to carbon cycle feedbacks, questions remain as to how both terrestrial and marine ecosystems, as well as ocean chemistry, will respond to human-caused perturbations in climate and CO2 levels.
At present only about half of the fossil carbon emitted by human activity stays in the atmosphere – the rest is absorbed, in roughly similar quantities, between oceans and ecosystems. This represents a negative feedback.
But the ability of these systems to continue to do so is limited. Rapid changes in climate affect the balance between import (via plant photosynthesis and growth) and export (via breakdown of organic matter and respiration of CO2 by animals, plants and micro-organisms) of carbon. Heat and water stress (eg, drought conditions), recurrent insect or disease outbreak or changing fire regimes may cause net transfers of terrestrial ecosystems carbon to the atmosphere.
Changes in the thermal structure and thus rates of overturn of the upper layers of the ocean also affect its ability to continually absorb atmospheric carbon.
The potential consequences of imbalances can be appreciated by considering the magnitude of current exchanges in terrestrial ecosystems. Human activity put an average of 7.2Gt (billion tonnes) of CO2 into the atmosphere annually between 2000 and 2005.
The annual exchanges between ecosystems and atmosphere are close to ten times this (but import and export are currently approximately balanced, or, as mentioned, slightly leaning towards ecosystem uptake). The total terrestrial carbon pool (soil and plants) is about 2,000Gt. Ocean exchanges are larger again, the total carbon pool nearly 40,000Gt.
A range of carbon feedbacks operating in these systems, both positive and negative, are likely to be triggered by warming, but the consensus is that the compound effect will likely “… reduce land and ocean uptake of atmospheric carbon dioxide…” (p11).
Improved understanding of key feedbacks has seen many of these processes incorporated into models (reflected in the upward revision of AR4’s upper temperature boundary), but uncertainties remain. The likely range of end of century temperature change across a range of scenarios is 1.1–6.4oC, but there’s the possibility, based on underlying uncertainties (not limited to carbon cycle feedbacks), that the upper boundary could be significantly higher.