The paleoclimatic record shows it has happened before – and now two well-read researchers illustrate it is already happening again: species across life’s kingdoms are decreasing in size, due to warming, droughts and acidification.

It’s a sign ecology is feeling sick – bad enough, poor little things. Smaller fish and smaller potatoes could also be a little hindering trying to feed the 7 billion. If however the shrinkage serves as indicator of a general biomass decrease, the worst bit of the bad news is the positive climate feedback this seems to pose. Don’t underestimate the amount of carbon nature cycles. (Disturb one single breath of it and those overworked IPCC contributors will have to rethink their 21^st century emissions scenarios all over – again.)

Five degrees warmer, half the size?

During the Paleocene-Eocene Thermal Maximum (PETM) temperatures on Earth increased somewhere between 5-10 degrees (over a period of 10,000-20,000 years). Palaeontologists have discovered that during this historic warming episode many species shrunk considerably in size. Mammals were on average around 40 percent smaller and insects and arachnids even decreased by 50-75 percent.

Now a metastudy by two researchers of the National University of Singapore of available peer-reviewed literature, published on Sunday in Nature Climate Change, shows the same process is already underway under the current warming – and not just for animals, but apparently also for plants.

Cold means big, climate change means malnourished?

Polar bears are the largest of bears, Siberian tigers are the largest of tigers, Arctic wolves are the largest of wolves. Somehow the average body size of animals increases to adapt them to living in harsh, cold climates.

This mechanism of biology however does not seem to be responsible for the opposite development: species shrinking under warming.

That’s because another explanation makes far better sense. Many species don’t reach their intended body sizes when somehow their growth is limited by a smaller supply of nutrients or disturbed by episodes of drought.

And the mighty creatures of the High North are not immune to this. The metastudy for instance showed that the average body mass of male polar bears over the last few decades has decreased by 11 percent. Between 1982-2006 their skulls shrunk 1 percent and their bodies grew 5 percent less tall.

Aquatic life forms shrinking due to droughts, warming, calcification disturbance

When encountered with spring droughts, like the one Europe experienced this year, the offspring of common toads and common frogs grows respectively 16 and 25 percent smaller. Many other amphibians and ectotherms [which may have a hard time adapting to elevated night time temperatures] suffer similar declines – the assembled literature shows.

Many commercial fish, like Atlantic salmon, herring and the orange roughy, have all decreased in size over recent decades, according to different climate-specific studies [so without influence of overfishing].

Warming, hypoxia and acidification are meanwhile responsible for a witnessed decline in average size of many shellfish, coral, shrimps. [We wonder how much this could contribute to the witnessed krill mass decline.]

But what happens when plants too shrink?

Perhaps most disturbing conclusion from the metastudy is that observed species shrinkage is not limited to the Animal Kingdom. Plants seem to grow smaller in a warmer climate too.

This could have important consequences for the human food supply – and for the global climate, as of course the entire carbon cycle could be influenced. A warming-induced general biomass decrease is one of the most terrifying positive climate feedback one can imagine – as the annual biological CO2 pump is still orders of magnitude larger (but so far in balance) than the anthropogenic emissions.

To illustrate the risk for agricultural productivity, the study shows strawberries in a 30 degrees Celsius summer produce 64 percent less fruit mass than in a 23 degrees summer. In a 6 degrees hotter summer maize production declines by 27 percent.

To illustrate the carbon cycle risk, the researchers have looked at tree species in the Amazon, South East Asia and Alaska, all of which show growth decline after droughts – which can be new to the ecosystems. Asian tropical trees can produce up to 80 percent less biomass during a dry year, the researchers show.

That could mean a big difference to the atmosphere. In the Amazon one record dry season can mean gigatonnes of extra CO2 – a modern case study we often cite. That means a drought a year does more damage than all current UNFCCC emissions reductions combined. Paradoxically, if the UNFCCC route fails, we’ll have many more of such droughts.

© Rolf Schuttenhelm | www.bitsofscience.org