For decades, it has been getting drier in some parts of Central America that have traditionally enjoyed plentiful tropical rains. Some University of New Mexico researchers say that is because of air pollution during the industrial age causing uneven warming of the northern and southern hemispheres.

They and their colleagues on an international science team say the uneven heating of the northern and southern hemispheres is nudging the inter-tropical convergence zone, where tropical rains form, to the south.

“What we’ve noticed basically over the last 500 years, especially since the advent of the industrial age, is that the region has been getting drier and drier, relatively,” said Professor Yemane Asmerom of UNM’s Earth and Planetary Sciences.

The historical evidence has been found during years of research in a remote cave in the Central American country of Belize, located right at the edge of the inter-tropical convergence zone.

The UNM team says the stalagmites of Yok Balum Cave have an especially accurate record of rainfall for the past 500 years.

“We’re able to see things we were never able to see before in the paleoclimate record,” said UNM Anthropologist Keith Prufer.

By studying the climate record in the cave, the ruins of sometimes ancient villages and historic human remains, Prufer and colleagues have been able to piece together the story of how changes in rainfall affect the people of the area who depend on it.

“Rain is necessary for agriculture and people to be able to live,” said Prufer, “So even a slight change in the amount of rainfall can have a lot of implications for food production in the region.”

The UNM team takes pieces of stalagmites to their lab in New Mexico.There, an advanced mass spectrometer is used to measure the traces of radioactive materials in different layers of the stalagmites.

“The more uranium we have in a sample, the better we can do,” said UNM research scientist Victor Polyak, “These samples from Belize contain a fair amount of uranium, and that’s allowing us to date the sample over the last 500 years very accurately.”

The UNM team says what the cave samples reveal is that since 1850, as modern man began dumping more aerosols into the atmosphere, primarily in the northern hemisphere, tropical rains began to drift south, away from Belize.

In a paper for Nature Geoscience, the international team concludes that the aerosols pumped unevenly into the northern hemisphere created a shielding effect in that hemisphere.

“The northern hemisphere has been warming at a lesser rate than the southern hemisphere because we’ve been putting these aerosols in the northern hemisphere,” said Asmerom.

The team believes the southward pressure on the inter-tropical convergence zone is created by the human-caused uneven hemispheric heating.

The scientists say they can see the effects of natural pollution like volcanoes in the cave samples.

“We were able to see how volcanic events affected our record,” said Polyak, “This was important because it helps us distinguish the natural climate change, from the climate change that is resulting from the injection of aerosols into the atmosphere.”