We all like to be warm, particularly at this time of year but, for animals, when does cosy turn into dangerous heat stress?
Imagine the first minute or two on a classic rollercoaster. There is a long, steady incline as the cars get pulled up the track, followed by a brief peak and then a sharp, stomach-churning drop.
The shape of the roller-coaster track governs how life processes, from bacterial growth rates to a lizard’s running speeds, respond to temperature.
When it is cold outside, biology is slow. As temperatures increase, so too do biological processes like cell chemistry, reaching a peak or optimum where life hums along at top speed.
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When temperatures increase beyond the peak, there is a sharp drop in performance down to a minimum or catastrophic level.
These responses of biology to temperature lead to the asymmetric or roller coaster shape of the Universal Thermal Performance Curve.
A recent paper by Arnoldi and co-authors explores the fundamental mathematics that underlie most biological responses to temperature.
The shape of the Universal Thermal Performance curve means that as species increase, the rate at which they perform at higher temperatures, they get less tolerant of hotter temperatures, and the roller coaster drop gets steeper. There can, therefore, be a remarkably narrow band of temperatures, above the optimum for each species, which are compatible with species performance.
Performance in this case covers a wide range of regular activities, such as swimming speed in fish, photosynthesis in plants, or cell division in bacteria.
This means that as the global climate warms, species will be at risk of being exposed to the dangerous side of their own particular temperature curve. They will be above their optimal temperature, with very little room for manoeuvre before facing the drop.
So this is all well and good in theory, but what kinds of warming are animals experiencing right now? An analysis by Merow and co-authors of the impact of 2024, the hottest year on record globally to date, showed that from a sample of 33,000 animal species, one in six were exposed to unprecedented warm temperatures across a quarter of the total area they occupy.
This did not just affect the rare species restricted to small areas and with specialist requirements for narrow temperature ranges. The research showed widespread species, those commonly found across a wide range of environments, were exposed to extreme temperatures across more than 10 per cent of the area they occupy.
Humans are not immune to the effects of increasing temperature. Lakhoo and colleagues showed that heat exposure poses a big threat to the health of mothers and newborn babies.
They found a very high certainty that preterm birth was related to heat exposure and a high certainty that gestational diabetes, hypertensive disorders of pregnancy, stillbirths and neonatal admissions were also related to heat exposure.
This evidence came from 33 countries, the majority of which, like Ireland, have high incomes and are in a temperate climate zone.
These observed effects of heat on maternal and newborn health are therefore likely to underestimate the effects of increases in temperature in lower-income countries, where vulnerable populations may not have access to protective features such as adequate shelter or sophisticated health systems.
Climate change is not a future problem. We are already seeing quite profound impacts of small temperature increases on animals throughout large proportions of the areas they occupy and on human health and wellbeing.
Mathematical and biological theory tells us that there is an upper and increasingly narrow limit to the ability of life to adjust to higher temperatures.
Meanwhile, we see that global carbon emissions from fossil fuels are projected to have risen by over one per cent in 2025.
As the climate warms, it weakens the ability of land and ocean systems to absorb carbon and lock it away in soils, ocean sediments, trees and peat.
Every ton of carbon that we prevent going into the atmosphere matters right now; every fraction of a degree that we save is important. We need to flatten out the path of the roller coaster we are on.
- Prof Yvonne Buckley is Co-Director of the Co-Centre for Climate + Biodiversity + Water and professor of Zoology at Trinity College Dublin
