Article
Delworth, T.L., Zeng, F., Rosati, A., Vecchi, G.A., Wittenberg, A.T., (2015), A link between the hiatus in global warming and the North American drought, Journal of Climate 28, 3834-3845, doi: 10.1175/JCLI-D-14-00616.1
Background
Flip on the evening news and you will likely hear about the drought in western North America. Although the drought has been in place for a decade, we have hit a major turning point where the daily lives of people living in California have been disrupted through mandatory water restrictions. Tune in to certain radio or cable news stations, particularly those with strong political agendas and you may find yourself on the receiving end of a diatribe against climate scientists with arguments of global cooling (read: warming hiatus) over the past decade. The correlation between these two climate irregularities should stir up the scientist within us all. Could there be a causality of these two news-worthy topics? Are we to blame, or are these phenomena the result of natural variability?
The term warming hiatus refers to stagnation in global mean surface warming over the past decade. The cause of the warming hiatus has been explored and linked to increased speeds of the Pacific’s easterly trade winds and cooling of the surface of the eastern Pacific ocean over the last decade. For more on how increased winds and reduced surface temperatures in the Pacific Ocean and the connection with the warming hiatus, have a look at the coverage of England et al., 2014 . Scientists have also made connections between increased easterly trade winds in the Pacific to other ocean basins. Check out McGregor et al., 2014 to dive in deeper!
Methodology
This study aims to add to the body of knowledge of the relationships between the increase in Pacific easterly trade winds and global climate using climate modeling experiments. The purpose of climate models is to best represent the physics of Earth’s atmosphere and oceans to answer questions about what the future climate will bring. For example, scientists seeking to understand how increased wind speeds impact sea surface temperature can use climate models to explore this relationship. Using these models, scientists can simulate future increases in equatorial winds, and observe how sea surface temperature responds in the coming decades. Scientists consider models to be informative if the models are able to predict climate conditions over a period of time in which accurate observational data is available for comparison. For example, a good model using accurate greenhouse gas concentrations will spit out global temperature trends over the 20th century that are consistent with observations from weather stations over the same time interval. Of course, this example is very simplistic to demonstrate how climate models can be useful for making projections of climate into the future.
These images show the spatial changes in temperature from 2002 – 2012 resulting from increased easterly trade winds. Cool colors represent decreasing surface ocean temperatures, whereas warm colors represent increasing temperature.
Results
Redundancy in science can be a good thing: the results of this study largely agree with those of England et al., 2014. Delworth et al. report that the modeling simulations show the same hiatus in global surface temperatures over the past decade in response to the increase in Pacific easterly trade winds (Figure 1). They also acknowledge that there have been studies that have linked cool eastern Pacific sea surface temperatures with drier than normal conditions over western North America (Figure 2). The remaining question was whether the changes in precipitation were due to anthropogenic climate change (the increase in atmospheric concentrations of greenhouse gases) or to the changes in wind strength in the equatorial Pacific. By separately considering the drought response to changes in winds and the drought response as a result of increased greenhouse gases, the authors were able to attribute 92% of the reduced precipitation to changes in winds. This result strongly connects the North American drought to the warming hiatus and increased trade winds. For this reason, the North American drought cannot be considered a response to anthropogenic climate change, just as the warming hiatus is due to natural variability in the climate system.
Colors represent the daily precipitation in mm/day, and averaged over the course of a year. Warm colors represent very low precipitation and drought-like conditions. Observations of precipitation are shown in (a), whereas precipitation from modeling simulations are shown in (b,c,d).
Significance
So are humans off the hook for climate change? Are SUVs back “in” and hybrids destined for the junkyards? It is refreshing to consider that we are not responsible for the lack of precipitation, but by no means should this be a call for celebration. I’ll leave you with the latest IPCC assessment report if you are not convinced. One may ask: Well shouldn’t the drought come to an end as the warming hiatus wanes? The models predict that the warming hiatus will indeed come to the end, having only lasted between one or two decades, though ocean temperatures will remain persistently cool over the eastern Pacific (Figure 3). As a result, drought conditions are still expected to persist over western North America, but at a level that is not quite as severe. Put away those balloons and noise-makers!
Despite the return of surface air warming (a), cool eastern equatorial Pacific surface temperatures persist (b) and drought-like conditions continue in western North America (c).
How has the North American drought affected you? Is it reassuring that we aren’t the cause? Sound off in the comments below!
I am a recent graduate (Dec. 2015) from the University of Rhode Island Graduate School of Oceanography, with a M.S. in Oceanography. My research interests include the use of geophysical mapping techniques in continental shelf, nearshore and coastal environments, paleoceanography, sea-level reconstructions and climate change.
