Air pollution pumped out by factories and power plants in Europe and North America has led to drier spells in the tropics, thousands of miles to the south. Scientists had long suspected this was the case and even had modelled the change in computer simulations, but now for the first time we have direct evidence – straight from a cave in Belize.
Most of us, when asked to think about climate change, think of global warming and the unequivocal rise in greenhouse gases. But greenhouse gases aren’t the only pollutants we produce which have the potential to disrupt the climate.
Atmospheric sulfate and nitrate aerosols, produced from burning fossil fuels, alter the climate both directly by reflecting solar radiation and indirectly by altering clouds. The impact of these aerosols is to offset the warming caused by greenhouse gases – where GHGs cause the Earth to retain heat, aerosols keep heat out in the first place.
However, as aerosols don’t stick around in the atmosphere for long, their effects are much stronger close to their source. It’s one reason why we see big regional differences in climate change. Indeed, observational and modelling studies provide considerable evidence that aerosols have lowered surface air temperatures in the northern hemisphere, offsetting greenhouse warming.
Understanding past climate
Characterising our influence on the climate is challenging for many reasons. Climate is a complex web of intricately bound variables, difficult to understand and even more difficult to predict. But there is one issue with climate science that complicates things more than any other: the lack of instrumental data.
Beyond the past 130 years, instrumental and observational data is sparse and uneven. It’s not very useful when looking to give context to our current climate debate. The study of climate history therefore relies on proxies to reconstruct the conditions at a given point.
These proxies are natural archives such as sediment cores, ice cores, tree rings and rocks – these record certain aspects of the climate in their physical characteristics. The width of tree rings, for instance, or the amount of carbon found inside air bubbles trapped in Antarctic ice for thousands of years. By analysing these archives we can create climate records that extend far beyond the short era of thermometers and rain gauges.
In our research, published in Nature Geoscience, we present one such climate reconstruction produced from a stalagmite collected from a cave in Belize in Central America. Stalagmites (the ones that grow upwards from the ground) grow incrementally as saturated water, filtered through the rock above, drips into the cave and leaves behind what becomes new rock.
Every drop of water has a unique chemical signature that is largely controlled by prevailing climate conditions above the cave, meaning that stalagmites record climate changes as they grow. By analysing the geochemistry of these incremental growth layers in a 450 year-old stalagmite, we were able to construct a historical rainfall record for the region.
The impact of aerosols on tropical rainfall
Recently it has become increasingly clear that climate changes in one region can have an impact in a totally different latitude. The IPCC’s 2013 summary for policymakers IPCC 2013 concludes with confidence that man-made changes in the North Atlantic climate are linked to rainfall at lower-latitudes.
Precipitation in the tropics, including Belize, is governed by the Intertropical Convergence Zone (ITCZ) – a belt of monsoon rainfall encircling the Earth near the equator that migrates seasonally between the hemispheres. The relative temperature difference between the hemispheres plays a crucial role in controlling the position of the ITCZ and hence, rainfall distribution in the tropics.
What we found was a distinct drying trend in Belize since 1850 that coincides with a steady rise in industrial aerosol emissions in North America and Europe. This presents strong evidence that industrial sulfate emissions have shifted the position of the ITCZ through reflecting the Sun’s incoming radiation and therefore moderating warming in the northern hemisphere.
In other words, aerosol pollutants have changed the relative thermal contrast between the two hemispheres and subsequently led to the ITCZ moving southward. This means less rainfall for the northern tropics. The role of sulphate aerosols in repositioning the ITCZ was previously identified through computer modelling techniques, but until now no suitable climate record existed to support those ideas.
Our claims are backed up by the volcano record. Emissions from volcanoes are similar to those produced by burning fossils fuels – basically lots of sulphur – and we identified short-lived drier spells in the northern tropics following very large volcanic eruptions in the northern hemisphere, such as the Icelandic Laki eruption in 1783.
This provided evidence that any injection of sulphate aerosols into the upper atmosphere, both natural and man-made, can disrupt temperatures and rainfall. These volcanically forced dry periods essentially rule out the possibility that the climate shifts were caused by a previously unknown natural climate cycle or increasing atmospheric carbon dioxide concentrations.
Although warming due to man-made carbon dioxide emissions has long been at the centre of discussions regarding climate change, the shifting of rain belts has significant regional-scale effects. The tropics are heavily populated and extremely reliant on regular rainfall. Linking human-induced changes and natural changes from the past to understand where the climate currently stands, and where it might go in the future, will be as socially important as it is scientifically challenging.