Four degrees and beyond: the potential for a global temperature increase of four degrees and its implications
Following up on Freetoken’s post, this special issue from the Royal Society needs to be read and understood by everyone. I will let the papers speak for themselves.
Beyond ‘dangerous’ climate change: emission scenarios for a new world
If government responses to climate change are to be evidence-based or at least informed significantly by science, the argument for low probabilities is reinforced still further. The characterization of 2◦C5 as the appropriate threshold between acceptable and ‘dangerous’ climate change is premised on an earlier assessment of the scope and scale of the accompanying impacts. However, these have since been re-evaluated with the latest assessments suggesting a significant increase in the severity of some impacts for a 2◦C temperature rise (e.g. [20,21]). Consequently, it is reasonable to assume, ceteris paribus, that 2◦C now represents a threshold, not between acceptable and dangerous climate change, but between dangerous and ‘extremely dangerous’ climate change; in which case the importance of low probabilities of exceeding 2◦C increases substantially…
…From a mitigation perspective, the gap between the scientific and policy understanding of the challenge needs urgently to be addressed. What is perhaps less evident is the implication of this gap for adaptation. As it stands and in keeping with the dominant policy discourse, the framing of much of the detailed research and practice around adaptation, if guided quantitatively at
all, is informed primarily by the 2◦C characterization of dangerous climate
change. Yet, as the impacts of rising temperatures are unlikely to be linear and also given rising temperatures are increasingly likely to be accompanied by additional feedbacks and hence further temperature rises, adaptation must consider more extreme climate change futures than those associated with 2◦C.
(All emphasis mine)
Now compare that with these findings from the same journal issue:
When could global warming reach 4°C?
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) assessed a range of scenarios of future greenhouse-gas emissions without policies to specifically reduce emissions, and concluded that these would lead to an increase in global mean temperatures of between 1.6◦C and 6.9◦C by the end of the twentyfirst century, relative to pre-industrial. While much political attention is focused on the potential for global warming of 2◦C relative to pre-industrial, the AR4 projections clearly suggest that much greater levels of warming are possible by the end of the twentyfirst century in the absence of mitigation. The centre of the range of AR4-projected global warming was approximately 4◦C. …
…Using these GCM projections along with simple climate-model projections, including uncertainties in carbon-cycle feedbacks, and also comparing against other model projections from the IPCC, our best estimate is that the A1FI emissions scenario would lead to a warming of 4◦C relative to pre-industrial during the 2070s. If carbon-cycle feedbacks are stronger, which appears less likely but still credible, then 4◦C warming could be reached by the early 2060s in projections that are consistent with the IPCC’s ‘likely range’.
Now consider some of the effects of this:
Regional temperature and precipitation changes under high-end ( ³4°C) global warming
Forty global climate model projections using the A2 scenario from the IPCC Fourth Assessment Report have been analysed, and a number of simulations that project a high-end warming of 4◦C or more by the 2090s (relative to the preindustrial period) were found. About half of the simulations were classed as high-end. Under the high-end models, Northern Africa is projected to experience high (greater than 6◦C) temperature increases and large precipitation decreases in both DJF and JJA, suggesting that this region is most at risk from high-end climate change. In addition, during JJA, Southern Europe and the adjacent part of Central Asia are projected to warm by 6–8◦C, together with a decrease in precipitation of 10 per cent or more. This result suggests that drier soils, a consequence of the reduced precipitation, are the cause of the elevated temperatures, as the evaporative cooling effect will be smaller.
Rethinking adaptation for a 4°C world
Humanity’s ability to adapt physically to a 4◦C world will depend in part on how well people adapt psychologically. Governments, other organizations and individuals will not undertake adaptation activities until they accept the need to do so. Most adaptation literature assumes that accepting the need to act follows from demonstrating the damage that will flow from failing to act. This makes the unwarranted assumption that humans respond to threats with adaptive coping strategies rather than with psychologically maladaptive ones or forms of denial .