World Bank Climate Predictions

by Duncan Gromko

With President Obama’s announcement of new climate change action on Tuesday, the World Bank has just released a report (written by the Potsdam Institute for Climate Impact Research and Climate Analytics) that projects how the climate will change over the coming century and how people will be affected. Spoiler alert: the news is pretty bad. How bad will depend on whether or not we get serious about limiting greenhouse gas emissions.

One big uncertainty is how much greenhouse gases humans will emit over the next century. The World Bank is basing its projections on “Representative Concentration Pathways” (RCPs), which are different scenarios for global greenhouse gas emissions levels and predicted impact on global temperature rise, shown below. RCP8.5 is the worst-case scenario, where there is no effort to decrease emissions. The other scenarios show moderate efforts to reduce emissions (RCP6 and RCP4.5) and successful effort to reduce emissions (RCP3).

If the world meets all the climate pledges it has currently made, there is a 20 percent chance of exceeding a four degree (centigrade) global temperature rise by 2100. If emissions continue as they are today, then the chances of exceeding four degrees by 2100 increases to 40%. Unless there is drastic and immediate reductions in emissions, it looks certain that we will top a two degree rise, with a reasonable chance of going over four degrees.

Compared to temperature rise, ocean acidification is something that gets much less attention. Considering the importance of coral reefs as nurseries for global fisheries, I think this is a huge oversight. As carbon is emitted into the atmosphere, the ocean also absorbs it, increasing ocean acidity. Under RCP8.5 and RCP6, there will be nearly complete die-off of coral reefs and a devastating impact on fisheries.

How will this affect us?

Temperature increase and ocean acidification will have many consequences for human well-being, including: rising sea levels, increased intensity of tropical storms, increased precipitation extremes (more droughts and floods), decreased agricultural productivity, and increase in vector-borne diseases (e.g. malaria and dengue fever).

One big takeaway from the report is how different parts of the world will be affected in very different ways. For instance, while drier conditions are projected for most regions close to the equator, including southern Europe, Africa, and much of North and South America, wetter conditions are projected for places at northern high latitudes: northern North America and northern Europe. Monsoon rains are expected to be more intense. Amazingly, even sea level rise is not uniform; in general, equatorial regions are likely to have higher sea level rise.

Close to the equator, crop productivity is expected to decrease, but at most northern latitudes, productivity will increase. Given that tropical countries tend to have the most food insecurity, this is just about backwards from the way you’d like the changes to happen. Globally, food production will increase as long as temperatures stay below a three degree increase, but at that point productivity will decrease.

But it’s not just the physical change that will be different across different world regions. Current and future wealth and investments will play an enormous role in who is most impacted by climate change. Wealthier nations will have a much easier time adapting to climate change. Wealthy cities like New York City can build sea walls to delay the negative impacts of sea level rise and increased storm intensity, but Dhaka Bangladesh will have a much harder time adapting, likely leading to millions of climate refugees.

Uncertainty

Even beyond all the uncertainty about emissions and how they will impact the climate, the models used in this report cannot capture “non-linear” events. There are feedback cycles, tipping points, and interactions between different variables that are just about impossible to model precisely. The problem is that non-linear events are exactly the kind you’d want to be able to predict. In the financial world, it was a similar kind of tipping point that ended up throwing the world into a financial crisis in 2007 and 2008.

The non-linear effect that is nearest and dearest to my heart is Amazon rainforest die-back The rainforest is so enormous that it generates and recycles much of the rain that falls in the Amazon basin. As deforestation continues, decreased tree cover may lead to less rainfall. A 2010 drought, which led to the death of billions of trees and an estimated release of 2.2 billion tons of greenhouse gases, may have been caused by decreased tree cover in the rainforest. Deforestation leads to drought, which leads to more deforestation. On top of this feedback loop, increased temperature in the Amazon due to climate change will exacerbate this effect: “there is a significant risk that the rain forest covering large areas of the Amazon basin will be lost” due to climate change. The Amazon is currently a global carbon sink, absorbing more carbon than it emits. If this die off occurred, it would very quickly become an enormous source of greenhouse gas emissions and completely outside of our control to stop.

This is just one example of a very harmful “non-linear” system and there are many others. The challenge for the World Bank and other policy and investment planners is that they would like to take steps now to adapt to climate change. While the projections in this report and other modeling can guide adaptation, the uncertainty makes it hard to plan. We know, for instance, that coastal cities will need protection when sea-levels rise. But should we plan for 1 meter of sea-level rise (the median estimate for 2100 under four degree warming) or more?