Turn down the heat : confronting the new climate normal is a massive 320 page report prepared for the World Bank by the Potsdam Institute for Climate Impact Research and Climate Analytics, and hence highly authoritative. The lead author was Hans Joachim (John) Schellnhuber of the Potsdam Institute.

From the Foreword:

There is growing evidence that warming close to 1.5°C above pre-industrial levels is locked-in to the Earth’s atmospheric system due to past and predicted emissions of greenhouse gases, and climate change impacts such as extreme heat events may now be unavoidable.

As the planet warms, climatic conditions, heat and other weather extremes which occur once in hundreds of years, if ever, and considered highly unusual or unprecedented today would become the “new climate normal” as we approach 4°C—a frightening world of increased risks and global instability.

The consequences for development would be severe as crop yields decline, water resources change, diseases move into new ranges, and sea levels rise. Ending poverty, increasing global prosperity and reducing global inequality, already difficult, will be much harder with 2°C warming, but at 4°C there is serious doubt whether these goals can be achieved at all. (Emphasis added)

Climate Progress has an excellent short summary. I’d like to comment on several aspects of the report which looked at climate impacts under three scenarios – the present climate of 0.8°C, a 2°C world and a 4°C world. The report focusses on three regions in considerable detail:

The report finds that overall 1.5°C warming is likely to be reached by 2050. If no mitigation action is taken we have a 40% chance of exceeding 4°C by 2100. If all countries that have made mitigation pledges follow them we can expect a median temperature rise of 3.1°C above pre-industrial.

In this the report is following the work done by UNEP, albeit in 2013. The 2014 UNEP report comes to similar conclusions. This should be seen as a critical endorsement of the UNEP report. Had they disagreed they would have said so.

Tipping points

Importantly Turn down the heat emphasises that if we breach 4°C the temperature is unlikely to stabilise at that point and will probably warm to 6°C or more. For a stable climate warming must be held below 2°C, because at that point significant tipping points could kick in.

For example in a 2°C world, the thawing of the Russian permafrost is projected to increase methane emissions by 20–30%.

Then the report looks in considerable detail at research on the Amazon. The short answer is that we don’t know with any precision what the impacts would be, but die-back and loss of forest cover could be a significant and increasing factor from about 1.8°C. At 4°C some studies see up to an 80% loss of forest cover. Other studies are less pessimistic.

What is not immediately apparent is that local temperatures will rise significantly more that global temperatures. A 4°C world could see local temperatures in the Amazon up to 10°C higher. A number of factors are at work.

Firstly, about 70% of the earth’s surface is ocean which warms about half as much as the land. Then as the forest warms and more droughts occur, there is die-back and an increased risk of fire. Without forest cover, warming increases. Also there is less rain as 28 to 48% of the Amazon rain comes from evapotranspiration of the forest. Hence dryer and warmer. In this way feedback loops are set up leading to further loss of carbon absorption capacity to the point where the Amazon could become a carbon source rather than a sink.

Crop yields

One of the concerns with warming is the change in crop yields. Here are some examples from Latin America and the Caribbean:

I’m surprised at how soon the effects kick in and how much they vary from one country to another. Obviously the news is not all bad with rice yields gaining. We can’t all grow rice, however, and the overall pattern is quite negative in terms of feeding a growing population and the social and economic disruption in the agrarian sector.

One of the problems with a warming world is more extreme weather, so more hot spells, droughts and floods. Life on the land becomes more uncertain as a result.

Sea level rise (SLR)

Sea level rise differs significantly in different coastal areas and the report documents expectations in detail. Overall they see greater rise by 2100 than the PCC report, because recent research not taken into account by the IPCC underestimates the fragility of the ice sheets. However, the difference is not substantial. In both cases sea level rise is seen as highly disruptive, as half to a metre will threaten many coastlines.

In both cases I find the long term projections surprisingly conservative. Over 2000 years they see current temperatures yielding 2.3 metres of SLR. At 2°C that becomes 3.6 and at 4°C a rise of 8 metres is forecast. I can’t get out of my mind Archer’s diagram of 2006:

Coming out of the last ice age, with 5-6°C we had 120 metres of SLR. According to the paleo record with a further 4°C we can expect all the ice to be gone and a further 75 metres or so. That makes sense in such a powerful way that I will continue to believe it until it is addressed directly and shown to be wrong.

An explanation might be that SLR happens slowly and might take 3-6000 years. True enough but then we are forcing the system many times (25 to 30?) harder than it has been forced before. There is no precedent. And there has been a recent precedent for rapid SLR 14,000 years ago in what is called Meltwater Pulse 1A, when the waters rose a metre every 20 years for 400 years:

There is no room for complacency.

Conclusion

This report brings home that 2°C is dangerous. We really need to try to stay well clear of it. In Climate change: reconnecting politics with reality Professor John Wiseman spells out the strategies we need to adopt for rapid decarbonisation for the Centre for Policy Development. As I said there, I’d go a little harder, but I think his approach is sound. Total renewable power generation within 10 year, for example, sounds good to me.