In this update:
Doomsday 1: Economic damages from climate change in the US
A landmark study published in the prestigious science journal Science in June this year, estimates the direct economic cost of climate change to the USA at roughly 1.2% of gross domestic product on average per 1°C of global warming. The study by Solomon Hsiang and 11 other scientists is based on a large number of empirical observations of spatially disaggregated direct economic costs in various sectors (see figure 1).
The cost is not evenly distributed across the USA. The poorest third of the counties, mostly located in the south of USA, will see damages between 2 and 20 per cent of county income under a business-as-usual scenario towards the end of the century. Figure 2 below shows a projection of county-level median damages towards the end of century.
If emissions growth is not slowed and we reach the projected 3-5°C of warming by the last two decades of this century, the USA will face costs on par with the Great Recession in the 1930s, or higher for poor regions – except they will not go away afterwards.
Doomsday 2: When the ice melts
Melting and transport of ice sheets and the resulting flow of freshwater into the oceans still represent a blind spot in our understanding of the climate system and projections about the future climate. In a relatively new study from 2016, James Hansen and co-authors warn that the impact of these flows on the future climate could result in multi-meter sea level rise.
The study takes a new approach: it describes freshwater flow into the North Atlantic and the Southern Ocean around the Antarctic and study the expected climate impacts, combined with physical reasoning around possible positive feedback mechanisms for such freshwater flows, paleoclimate observations and modelling.
Briefly, what happens is that the increased freshwater inflow stratifies the oceans by placing the freshwater as a cool top-layer, since freshwater is lighter than the normal salty ocean water. As a result, the normal overturning taking place in both northern and southern waters is reduced. As the heat can no longer escape to the atmosphere, this leads to warming of the deeper ocean. This will further melt ice sheets grounded below the ocean surface, allowing more ice to be transported from inland to the ocean. An initial pulse of freshwater will therefore create a positive feedback, securing ever more freshwater in the form of ice to enter the oceans. This could lead to exponential sea level rise.
The fact that heat no longer can easily flow from the oceans to the atmosphere will furthermore cool the atmosphere. This is shown in Figure 3: Cool (blue) areas are observed in the North Atlantic and in the Southern Ocean. In fact, the simulation leads to a more or less complete shutdown of the North Atlantic Drift, cooling much of Northern Europe, even in a warmer world, as long as the enhanced freshwater inflow continues. Finally, the larger temperature gradient between these colder areas and the increased warming around them, can feed an increased frequency of ‘super storm’ – storm with enormous wind speed and huge amount of precipitation.
The lesson from all this? We are already observing increased freshwater inflow into the oceans from transport of inland ice. The climate might be more sensitive (due to the feedbacks) to this than we have previously thought. The consequences are such that that no reasonable adaptive behaviour can mitigate the economic and social damages expected from this. Thus, we need to reduce and hopefully eliminate our CO2 emissions as soon as possible. Or else …
Doomsday 3: Food for thought
Climate change is projected to negatively impact all aspects of food security according to the IPCC. Usually, the potential impacts are studied for one region and one crop at the time. In a new study researchers from the Met Office of UK assess the likelihood of what they call “multi-breadbasket failure”, i.e. simultaneous yield failure in all important production regions for maize due to water stress.
They find a 6% chance per decade for large-scale, multi-breadbasket failure for this important crop. Moreover, they note that empirical estimates based on historical data are not necessarily a good guide for what can happen as the world heats up.
A study by the EU Joint Research Centre (JRC) takes a closer look at another important crop – wheat, which stood for approximately 20% of worldwide dietary calories in 2010. The researchers studied how heat, drought or water excess affected wheat production in different regions.
They find that 40% of global wheat production variability can be explained by these factors. Perhaps surprisingly, in some countries, e.g. France, excess water is a bigger problem for wheat productivity than drought. With better seasonal weather forecasts, the method used in this study can allow for better adaptation measures for wheat production.
The take-home message is that food production is going to be more challenging in the future, so expect higher food prices.