Drought in the context of anthropogenic climate change

In the previous post I talked about European droughts association to natural modes of variability. Since this is a blog about natural versus anthropogenic climate change I thought it would be natural to follow up with a question about how anthropogenic influence on climate is affecting droughts. In particular, considering the severe drought in the US the last summers, this question is of high relevance.

A relatively newly released study in Nature by Sheffield et al. look closer at the historical record of global-scale drought trends and actually find that they have most likely been overestimated. They report high uncertainties in these trends over the past 60 years and little evidence of an increase in the total area affected by drought. One might think that it is the opposite way around in these times of a warming climate, but the climate system is highly complex and we don’t necessarily get the results that seem most logic. There exists a hypothesis in the scientific world saying that “wet is getting wetter and dry is getting drier”, meaning that the areas which normally receive a lot of precipitation will get more intense rainfall and flooding, and that areas that are already suffering from a precipitation deficit and drought will get more severe droughts. This will have huge consequences for people living in such areas.

Another study produced by a big group of scientist for the American Meteorological Society looks at six extreme events during the year of 2011 and tries to explain them from a climate perspective. Among these, they look at the severe 2011 Texas drought and ask: “Was the likelihood of either the heat wave or the drought altered by human influence on global climate?” Now considering drought over the North American continent, it is important to point out that ENSO with its cold phase La Niña, is considered to be a key driver of drought conditions in the central US (Atlas et al. 1993). In the study they use the La Niña year of 2008 as a proxy for 2011, because simulations under 2011 forcing conditions were not available, and compare to earlier decades. They find “that extreme heat events were roughly 20 times more likely in 2008 than other La Niña years in the 1960s and indications of an increase in frequency of low seasonal precipitation totals.” These findings suggest that drought is more probable now than for 40-50 years ago.

Picture from Texas drought 2011, Google

This also contribute to strengthen the “dry getting drier” hypothesis by saying that the probability of the occurrence of drought in an already dry area like Texas, is more likely with global warming.

European drought’s relationship to global SST

So far on this blog I’ve described natural modes of variability across the North Atlantic basin like the NAO, the AMO and AMOC and their related climatic impacts. They are all associated with specific climatic patterns of temperature and precipitation across large areas, and I’ve mentioned and showed illustrations of these in my previous posts. In this post I will only focus on the drought impact and try to give a summary of the factors controlling droughts across Europe.

A study by Ionita et al. (2012) looked at variability of European summer drought and its relation to global sea surface temperature (SST) by using the Palmer drought severity index averaged over the European region (see figure below).

 

This time series show the strong interannual and decadal variability of European drought. The authors show that winter SST has a strong impact in determining drought variability over Europe in the upcoming summer through different large-scale teleconnection patterns. By the use of correlation analysis they reveal the existence of three coupled modes of summer drought pattern and winter SST anomalies with different timescales:

1.    The first coupled mode represents the long-term warming trend in global SST caused by anthropogenic greenhouse gasses, in addition to a tripole-like pattern in SST resembling the positive phase of the NAO.

2.    The second coupled mode is associated with an inter-annual SST pattern in the Pacific which resembles the cold phase of ENSO (La Niña) together with the decadal fluctuation in extratropical SST resembling the Pacific DecadalOscillation (PDO).

3.    The last coupled mode is associated with strong multidecadal variability in SST across the Atlantic basin which corresponds with the AMO, also for the interannual variability. In a previous post I described how the AMO exerts a strong influence on European summertime climate, including drought. As they write in the paper: “According to Briffa et al. (2009) the summers of 1921, 1976, and 1990 were among the driest in the last 250 years, all these dry summers occurring during a cold North Atlantic phase of the AMO.”

I’ve previously described the NAO- and the AMO’s impact of heat and drought on the European climate. It now turns out that drought across Europe is associated with four different modes of variability! (NAO, ENSO, PDO and AMO). Drought is not an easy thing to define given the complexity of the phenomenon, and there also exists several types of drought. So to say that drought across Europe is determined exactly by these four modes of variability is of course just looking at the big picture.