According to a recent press release from UC Berkeley’s School of Law:

Overheated: The Human Cost of Climate Change predicts a grim future for billions of people in this century. It is a factual account of a staggering toll, based on hard data […] “Climate change is the most important problem facing the international community in the 21^st century,” Guzman said.

Guzman's view is shared by many.

While I have not read Guzman’s book, I have read GiveWell’s summary of the IPCC, as well as notes on GiveWell’s conversations with climate change experts. Based on these, I’ve come to the tentative conclusion that while climate change is an important issue, it’s unlikely to be the most important issue, though there is uncertainty, owing to poorly understood tail risk. 

Potential Impacts according to the IPCC 

GiveWell recently wrote up a summary and review of some of the impacts of unmitigated climate change, as described by the Intergovernmental Panel on Climate Change's 2007 Fourth Assessment Report. GiveWell writes:

The report suggests that unmitigated climate change would have extraordinarily negative humanitarian impacts across all of the outcomes we looked at: hunger, water stress, flooding, extreme weather, health, biodiversity, and the economy. Successfully mitigating these negative impacts would carry vast humanitarian benefits. 

When looking at the range of possible futures outlined in the report, the bulk of the variation (in humanitarian terms) comes from variation in the level of assumed economic growth and adaptation, rather than variation in the amount of climate change. Of the outcomes we examined, only biodiversity is expected to be unambiguously worse off in the future as a result of both climate change and economic growth.

The most succinct summary of the expected impact is given by a GDP drop estimate:

Most recently, Stern (2007) took account of a full range of both impacts and possible outcomes. […] Using equity weights to reflect the expectation that a disproportionate share of the climate-change burden will fall on poor regions of the world increased their estimated reduction in equivalent consumption per head to 20%.

Such a drop would be highly undesirable, but far from catastrophic. World GDP has been growing at a rate of ~3%, so this corresponds only to a set-back of ~6 years.  Poor countries are expected to become richer regardless, and because of marginal diminishing utility, such a setback would carry less negative humanitarian impact than such a setback would if it occurred today.

Overly weak assumptions regarding adaptation? 

The IPCC considers what the impacts of climate change will be in 2050; a full 35 years away. My intuition is that over the course of the next 35 years, human society will adapt in such a way that the issues that the IPCC describes will have a smaller negative humanitarian impact than the IPCC suggests. I have not vetted the references given by the IPCC, and may be mistaken about their implicit assumptions, so my remarks should be taken with a grain of salt.  

To explicitly address some of the impacts discussed in the IPCC:

•  Crop productivity and hunger in Africa — The IPCC projects that in 2050, at least 208 million people will be at risk of hunger even without climate change: only ~ 3x fewer than today. New technologies for the production of starch could make the number much smaller.
  
  
• Floods due to rising sea levels — The IPCC mentions dikes and nourishment as possible adaptive responses, but doesn’t mention the possibility that people will move away from coastal areas as sea levels rise. Individual people and families might move away to avoid flooding. On the time scale of 5 years, people are usually unwilling to move, but over the course of 35 years, it might happen organically. Over time, more cities will be built, and people will migrate to them. Cities are more likely to be built in areas that are not prone to flooding, than in areas that are.
  
  
• Significant loss of species and biodiversity — Over the next 35 years, humans may make a concerted effort to preserve rare species by collecting them and housing them in zoos. (I recognize that this would have to happen well before 35 years elapsed in order to avoid the loss.)

There are also more general relevant considerations:

• Geoengineering — It may be possible to reverse or halt climate change via geoengineering. Geoengineering carries its own risks, but its potential would seem to be positive in expectation, although the situation is murky.
  
  
• Unpredicted technologies — Technological progress is unpredictable, and it could be that we can develop technologies that mitigate effects such as droughts, even if we can’t see these technologies in advance. There are potential unpredicted technologies that would make climate change worse, such as those that facilitate cheaper extraction of fossil fuel, but on balance, technological progress usually enables humans to solve problems more than to create them.
  
  
  Breakthroughs in genetic engineering, artificial intelligence and whole brain emulation could also radically alter human needs themselves.

The IPCC as overly optimistic?

Some people have voiced the concern that the IPCC is overly optimistic in its predictions of climate change impacts. This is corroborated by the recent paper Climate change prediction: Erring on the side of least drama, which reports that historic IPCC predictions of current impacts of climate change have erred on the overcautious side. But given that the IPCC represents a consensus, barring tail risk, it seems unlikely that the IPCC is overly optimistic by a huge margin. 

Tail risk

The biggest reason to be concerned about climate change is that there’s a small probability that the negative impact could be huge.

There is a danger of permafrost in the Arctic melting and releasing methane, creating a feedback loop where the release of methane, global warming, and the melting of the permafrost reinforce each other. This could increase global temperature by 3.5 degrees Celcius, greatly exacerbating all other impacts of climate change. Even if this doesn’t happen, it could be that climate models are wrong, and that the amount by which the earth temperature will rise is much greater than current models predict.

The potential impacts of a much larger increase in temperature than what the IPCC predicts are less studied. One starting point for reading on this is the 4degrees and beyond international climate conference.

About the author: I worked as a research analyst at GiveWell from April 2012 to May 2013. All views expressed here are my own.

Acknowledgements: Thanks to Nick Beckstead, Vipul Naik and Carl Shulman for helpful suggestions.

To be continued: I'll be writing up a follow up report about the implications of the projected impacts for effective philanthropy.