March 25, 2017, by Tim Radford
Jaw and teeth fossils from the cat-sized Arenahippus, ancestor of today’s horse, are much smaller from a period of dramatic warming. Image: University of New Hampshire
Fossils from around 56 million years ago warn of the dwarfing effect that dramatic global warming can have on mammal species.
LONDON, 25 March, 2017 –Scientists in the US have proved it yet again that, in a rapidly warming world, mammals face a diminished future.
In addition to the extinction threats resulting from loss of habitat, drought, floods, heatwaves and the climate shifts that will accompany global warming, mammal species that survive will become smaller as the world becomes hotter, according to a team from the University of New Hampshire.
They went searching for fossils in the Bighorn Basin of Wyoming, and gathered enough teeth and jaw fragments to build up a picture of how four species – including a precursor to the modern horse – became smaller with time during a bygone episode called the Eocene Thermal Maximum.
This was an episode 53.7 million years ago, when over an 80,000 to 100,000-year span, planetary temperatures rose by between 2°C and 3°C.
Climate scientists and palaeobiologists have delved into this before. They found that about 56 million years ago, there was a much more dramatic warming episode called the Palaeocene-Eocene Thermal Maximum, when the world heated up by 5°C to 8°C and stayed hotter for at least 180,000 years.
Mammal sizes during this period dwindled measurably − and left the evidence in the fossil record.
Then scientists began looking at the Bighorn Basin for evidence from a second, later warming, and confirmed that during this episode, too, the size of mammals shrank.
The response could be attributed to a number of possible reasons – vegetation change, for instance – but the favoured explanation is a simple matter of physics.
Smaller mammals lose body heat more efficiently than bigger mammals, because there is a greater ratio of skin to volume, and biological cooling systems have to work overtime in a warmer world. Natural selection would favour the smaller specimens in each group, and go on favouring them.
“ The hope is that it would help us learn more about the possible effects of today’s global warming ”
What matters in the most recent study in Science Advances journal is that the researchers this time had enough specimens to make comparisons between the two periods.
They could confirm that although their examples of Arenahippus pernix, the horse ancestor, shrank by 14% during that second thermal maximum, this size reduction was nothing like the 40% shrinkage in another close relative, Sifrhippus, two million years earlier, when the world became much hotter and for much longer.
“Since the temperature change was smaller, this suggests there may be a relationship between the magnitude of a global warming event and the degree of associated mammal dwarfism,” says Abigail D’Ambrosia, a doctoral student on the natural resources and Earth system science programme at the University of New Hampshire, who led the study.
Studies such as this matter because there is unequivocal evidence that, as a consequence of prodigal human combustion of fossil fuels, greenhouse gas concentrations in the atmosphere are rising − and, with them, average planetary temperatures.
Far more damaging
Global average temperatures could rise by 4°C or more just in this century if humans do not drastically reduce fossil fuel use.
This projected rise is far faster, and potentially far more damaging, than the rates of rise 56 million and 54 million years ago.
Scientists have already started to see the effects of warming on reducing the body mass of the goatlike chamois antelope in the European Alps, and agricultural scientists have started to consider the effects of warming on domestic livestock.
The message from Bighorn Basin is that global warming, for whatever reason, precipitates change in the living world.
“We wanted to see if this pattern is repeated during other warming events,” D’Ambrosia says. “The hope is that it would help us learn more about the possible effects of today’s global warming.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.