Human influence on joint changes in temperature, rainfall and continental aridity

Study key findings & significance

• The study identifies two distinct human fingerprints on global droughts patterns, and shows that global warming is the driving force behind:
  • the intensification of patterns of drought and extreme rainfall as well as overall increases in large-scale continental aridity since 1950, and
  • a northward shift in the Intertropical Convergence Zone (ITCZ) since 1975, bringing less rainfall to the western US and more to the Sahel
• It is the first time that scientists have found the human fingerprint on intensification of patterns of drought and extreme rainfall – also known as “wet-dry patterns”.
• The fingerprint on the northward shift in the ITCZ also points to a detectable human fingerprint on decreasing rainfall over the US, central Asia and southern Africa as well as increasing rainfall in the Sahel region of Africa, India and the Caribbean.
• The changes are “largely driven by multidecadal increases in greenhouse gas emissions."

Author quotes

We find that since 1950, human-produced greenhouse gases and particulate atmospheric pollution have influenced global changes in temperature, precipitation and regional aridity in two distinct ways. These two human ‘fingerprints’ are statistically identifiable in observations.

Céline Bonfils, lead-author and physicist at the Lawrence Livermore National Laboratory

After 1980, the northern hemisphere became warmer than the southern hemisphere. There were two reasons for this. First, pollution regulations reduced manmade aerosol emissions in North America and Europe. Second, the greenhouse effect warms the northern hemisphere, which is predominantly covered by land, faster than the southern hemisphere, which is predominantly covered by oceans. For both of these reasons, the tropical rain belt [ITCZ] shifted back northward after 1980, bringing less rainfall to the western US and more to the Sahel.

Céline Bonfils, lead-author and physicist at the Lawrence Livermore National Laboratory

Abstract

Despite the pervasive impact of drought on human and natural systems, the large-scale mechanisms conducive to regional drying remain poorly understood. Here we use a multivariate approach to identify two distinct externally forced fingerprints from multiple ensembles of Earth system model simulations. The leading fingerprint, FM1(x), is characterized by global warming, intensified wet–dry patterns and progressive large-scale continental aridification, largely driven by multidecadal increases in greenhouse gas (GHG) emissions. The second fingerprint, FM2(x), captures a pronounced interhemispheric temperature contrast, associated meridional shifts in the intertropical convergence zone and correlated anomalies in precipitation and aridity over California, the Sahel and India. FM2(x) exhibits nonlinear temporal behaviour: the intertropical convergence zone moves southwards before 1975 in response to increases in hemispherically asymmetric sulfate aerosol emissions, and it shifts northwards after 1975 due to reduced sulfur dioxide emissions and the GHG-induced warming of Northern Hemisphere landmasses. Both fingerprints are statistically identifiable in observations of joint changes in temperature, rainfall and aridity during 1950–2014. We show that the reliable simulation of these changes requires combined forcing by GHGs, direct and indirect effects of aerosols, and large volcanic eruptions. Our results suggest that GHG-induced aridification may be modulated regionally by future reductions in sulfate aerosol emissions.