Extremes of heat − and an extra helping of drought − have begun to change the planet in small, subtle ways, and will almost certainly continue the process of change, according to new research.
Bird species such as the Elegant Tern have begun to move north from the Gulf of California in Mexico, a species of ant that lives underground has shown it cannot take the heat, and the giant trees of the world’s forests may be at risk.
The link between any single extreme of heat and drought, and global warming as a consequence of the emissions of greenhouse gases from human burning of fossil fuels, is almost impossible to prove, but climate science has begun to show that, in general, heatwaves and drought become more likely as global average temperatures soar.
But the argument is not conclusive. Daniel Horton, research fellow in Earth system science atStanford University, California, and colleagues show in the journal Nature that extremes of temperature in Europe and North America could be linked to changes in atmospheric circulation and to the distribution of heat and water vapour in the atmosphere.
That still leaves open the question: is that because of some natural cycle, or a response to global warming?
Kevin Trenberth, Distinguished Senior Scientist in the climate analysis section at the US National Centre for Atmospheric Research, and colleagues argue in Nature Climate Change that this is hardly an either/or question.
Natural variability and human-induced climate change may both be at work in extreme events. So there could be other ways of putting the question. Given a flood, where did the moisture come from? Could it be linked to high ocean temperatures that in turn could be linked to human-induced climate change?
Put like that, it might be possible to make connections between the steady rise of carbon dioxide in the atmosphere and catastrophic events such as Superstorm Sandy in 2012, orsupertyphoon Haiyan in 2013.
But while humans argue, the natural world responds. Most of the world’s population of the Elegant Tern (Thalasseus elegans) has, historically, nested on a tiny island in the Gulf of California, in Mexico. Lately, the birds have arrived to nest as usual, but increasingly have departed without nesting.
It happened in 1998, in 2003, and then in 2009, 2010, 2014 and 2015. The population has expanded north to San Diego, Los Angeles Harbour and other places.
Ecology researcher Enriqueta Velarde, of the Universidad Veracruzana, Mexico, and colleagues report in Science Advances that the Gulf of California has been abnormally warm for the last 15 years. And when that happens, the supplies of nutrient – and the small pelagic fish that eat the nutrient – tend to decline.
“Whenever the terns perceive the conditions in the Gulf as inadequate to ensure successful reproduction, they move to alternative nesting grounds,” Dr Velarde says.
Gulls and terns can migrate when things are uncomfortable. And many species of ant – such as army ants, the nomadic, swarming predators of the central American jungles − can cope with temperature changes. But unexpectedly, at least one subset cannot, according to Kaitlin Baudier, a biologist at Drexel University, Philadelphia, and colleagues.
These are the army ants that dwell underground, theoretically insulated from surface temperatures but actually at risk. Turn up the heat, and they suffer.
She and her co-authors report in the Journal of Animal Ecology that micro-habitat seems to be closely linked to a creature’s thermal physiology.
“This shows us the ways that these species respond to a changing climate will be different depending on habitat type, and it is important to know that micro-habitat could be an indicator of heat tolerance,” says Sean O’Donnell, a biology professor at Drexel, and senior author of the report.
Law of fluid flow
But while gulls can fly away, and ants can march or die, trees can only stay put and sicken when the heat is on.
That is because a tree’s survival depends on water that has to be sucked up to a great height through solid tissue − and this process is therefore subject to a law of fluid flow formulated in the 19th century by a French engineer, Henry Darcy.
Nathan McDowell, a climate researcher at Los Alamos National Laboratory, New Mexico, and Craig Allen, a research ecologist at the US Geological Survey, report in Nature Climate Change that, under climate warming, Darcy’s law predicts “widespread forest mortality”.
They put the question: given that trees have the problem of hauling water to a great height through old wood, what will higher average temperatures do to established forests?
The ones most likely to die, they found, were those that were very tall, and that functioned with isohydral stomatal regulation, low hydraulic conductance, and high leaf area. Helpfully, the scientists supplied the translation in the next sentence.
“Thus, tall trees of old-growth forests are at the greatest risk of loss,” they say, “which has ominous implications for terrestrial carbon storage.”