Polar bears and other animals are shrinking due to the affects of global warming on their living conditions, according to a new study.
Researchers found that species from micro-organisms to top predators are now growing to smaller sizes because of global warming.Nearly 45% of the species involved in the review grew smaller over multiple generations, the study published in the Nature Climate Change journal said.
Those affected include polar bears, red deer, soay sheep, gulls, lizards and the common toad as well as many plants such as tiny plankton.
The changes could have far-reaching consequences, including a serious impact on human food sources because of smaller fish supplies and less reliable crops.
Dr David Bickford and Jennifer Sheridan of the National University of Singapore wrote: "The consequences of shrinkage are not yet fully understood, but could be far-reaching for biodiversity and humans alike.
"Because recent climate change may be faster than past historical changes in climate, many organisms may not respond or adapt quickly enough. This implies that species may go extinct because of climate change."
The changes could lead to smaller fish supplies
The pair looked back at scientific writings on the past effects of climate change and decided fossil records unambiguously showed marine and land organisms had become smaller as temperatures rose.
During a warming event 55 million years ago - often seen as a parallel for current climate change - beetles, bees, spiders, wasps and ants shrank by 50 to 75% over a period of several thousand years.
Mammals including squirrels and woodrats also diminished in size by about 40% during what is known as the Palaeocene-Eocene Thermal Maximum (PETM).
But the pace of current warming is far greater than its predecessor and the new research showed it has already started to shrink species.
The study examined 85 examples, of which 55% were affected. Of those a fifth had grown but four-fifths had become smaller.
The scientists were shocked because they had thought plants would increase in size due to the increased amount of carbon dioxide in the atmosphere.
"Many organisms may not respond or adapt quickly enough. This implies that species may go extinct because of climate change."Dr David Bickford and Jennifer Sheridan
Findings suggested that for every 1C increase in temperature, the metabolism of cold-blooded animals roughly increases by 10%.
This means they use more energy and therefore grow to smaller sizes.
Global average temperatures have risen by around 1C over the past century and experts predict they could rise by between 4C and 7C by 2100.
A major concern is that species will become extinct because they will not be able to adapt quickly enough.
It is also feared that animals and plants will shrink at different rates, which could have a destabilising affect on sensitive ecosystems.
The study said: "We do not yet know the exact mechanisms involved, or why some organisms are getting smaller while others are unaffected.
"Until we understand more, we could be risking negative consequences that we can't yet quantify."
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