Climate change threatens one in six species with extinction, study finds

The risk of Earth’s species becoming extinct will accelerate as global temperatures rise, new research shows.

After reviewing more than one hundred scientific papers, the study finds as many as 16% of plant and animal species on land and in the oceans would be under threat with four degrees of warming.

Climate change could even overtake habitat loss and degradation as the main cause of extinctions, the lead author tells Carbon Brief.

Extinction risk

The rate at which plants and animals are becoming extinct is now a thousand times higher than before humans inhabited the Earth.

Habitat loss is the principal cause of extinctions, as forests are cleared and urban areas expand. But a new study, published in  Science, suggests that climate change could soon become a key threat to species around the world.

A warmer world could have many  different impacts on plants and animals, not least by pushing temperatures beyond species’ physical tolerance. Shifting seasons can affect breeding patterns, and hot days may mean animals have less energy to search for food.

Changes to rainfall patterns may affect availability of water and freshwater habitats. These changes could conspire to influence how much food a species can access, and what predators and diseases it is exposed to.

The combination of habitat loss and climate change is likely to intensify their individual impacts on different species, Prof Joshua Lawler, who wasn’t involved in the study but who is an author of the latest Intergovernmental Panel on Climate Change (IPCC)  report, tells Carbon Brief:

“[H]abitat loss and fragmentation will make it harder for species to move to suitable climates, and climate change will drive human migrations and shifts in the distribution of cultivated lands which will, in turn, reduce habitat for species.”

In the new study, Prof Mark Urban from the University of Connecticut aggregates the results of 131 studies on extinction risk to give a global picture of the risks posed by climate change.

Exponential rise

The  current target for international climate policy is to limit global temperature rise to 2C above pre-industrial temperatures. Even with this level of warming, we can expect to lose around 5% of species, the study finds.

But as you can see in the graph below, the predicted extinction percentage increases as global temperatures rise beyond the 2C limit.

Predicted extinction rates from climate change rise with global temperature. Blue bubbles show individual studies, and their size shows how many species the study assessed. Source: Urban ( 2015).

At 3C of warming, the extinction risk increases to 9%. Under a high emissions scenario, equivalent to the IPCC’s RCP8.5 pathway, four-degrees of warming would threaten 16% of species.

This is an exponential rise, Prof Terry Root, a review editor of the Terrestrial and Inland Water Systems chapter of the latest IPCC report, tells Carbon Brief:

“Extinction risk is not going up in consistent steps as the temperature increases, but the risk is going up in larger and larger steps with each degree of warming.”

Regional risks

The study also considered how risks vary between the different species and habitats around the world. We’ve summarised these in an infographic below.

Predicted extinction rates from climate change by region and group. Credit: Rosamund Pearce, Carbon Brief, based on data from Urban ( 2015).

South America has the highest extinction risk at 23%, the study says, followed by Australia and New Zealand (14%).

These regions have a diverse mix of species that have narrow habitat ranges, the study says. This means the species are less able to migrate if their current habitats become unsuitable.

Extinction risks in North America (5%) and Europe (6%) are the lowest across the regions. The percentage at risk in Asia is 9%, but data is limited for the region and this estimate is only based on four studies, the paper notes.

As for which groups of animals and plants are at risk, the study suggests amphibians (13%) and reptiles (9%) are most threatened by climate change. Extinction risks are also higher for creatures and plants that are native to a region. They face a 6% greater risk from climate change compared to those that have been introduced from elsewhere.

These most at-risk groups of species tend to have small habitat ranges, the study says. As the Earth warms, species are likely to migrate in search of food or cooler temperatures. But for some this won’t always be possible, warns Urban:

“Mountaintop species, such as the American Pika, will find their preferred climates shifting beyond the tops of mountains that they inhabit.”

Causal links

Predicting extinction risks from climate change alone is tricky as humans can affect ecosystems in many different ways, such as by deforestation, pollution and introducing invasive species.

Part of the difficulty is that scientists look at past extinctions to make predictions for the future. To date there aren’t many documented cases of climate change causing extinctions. For example, the IPCC says there is medium confidence that recent warming caused the extinction of some species of amphibians in Central America, but generally there is very low confidence that other extinctions can be attributed to climate change.

This makes it a challenging to make the connection, Prof Georgina Mace, an IPCC author from University College London, tells Carbon Brief:

“We have weak understanding of the causal processes linking climate change to increased extinction rates.”

There are also limitations with the models that scientists use, Mace says. They don’t include how different species interact with each other, or how a species might evolve to cope with different conditions.

Scientists don’t know for certain if taking account of these factors will make extinction predictions more or less severe, says Mace. But the next generation of models should incorporate these factors, recommends Urban in his paper.

In the meantime, climate change-induced extinctions look likely to become increasingly apparent if steps aren’t taken to reduce emissions, he concludes.

Main image: American Pika on a rocky mountain top.