Climate Change and Wildlife: Range Shifts, Phenology, Coral Bleaching and Adaptation

Earth’s average surface temperature has risen by about 1.2 to 1.3 °C since the late nineteenth century, mainly because burning fossil fuels and clearing forests have increased the concentration of greenhouse gases in the atmosphere. 2023 and 2024 were the hottest years on record. For wildlife, climate change is no longer a future threat; its effects are measurable in almost every ecosystem on Earth.

What you will learn

How greenhouse gases warm the planet, and how we know carbon dioxide is rising
The four main ways climate change affects species: range shifts, phenology, physiology and extreme events
Which ecosystems are most vulnerable, from coral reefs to mountaintops
How species adapt, and what conservation can do

The greenhouse effect and rising carbon dioxide

Greenhouse gases such as carbon dioxide (CO₂), methane and nitrous oxide let sunlight pass through the atmosphere but absorb some of the heat radiated back by Earth’s surface, keeping the planet warmer than it would otherwise be. Without any greenhouse effect, Earth’s average temperature would be about −18 °C. The problem is not the effect itself but its rapid strengthening.

In 1958, the scientist Charles David Keeling began measuring CO₂ at Mauna Loa Observatory in Hawaii. His record, the Keeling Curve, shows a steady rise from about 315 parts per million (ppm) in 1958 to over 420 ppm today, with a small yearly wiggle as Northern Hemisphere plants take up CO₂ in summer and release it in winter. Ice cores show that CO₂ did not exceed about 300 ppm at any time in the last 800,000 years before the Industrial Revolution.

How climate change affects wildlife

1. Range shifts

As temperatures rise, many species are moving towards the poles and uphill to stay within their preferred climate. A global analysis in 2011 found that species were shifting their ranges on average by about 17 km per decade towards the poles and 11 m per decade uphill.

In the tropical Andes, trees, birds and frogs are moving upslope.
Mountain species such as the American pika, snow leopard and many alpine plants are running out of room at the top, a situation sometimes called the escalator to extinction.
Marine fish are moving faster than land species; North Sea cod and plaice have shifted northwards and into deeper water.

2. Phenology: changing timing

Phenology is the timing of seasonal events such as flowering, leaf-out, breeding, migration and hibernation. In many places spring now arrives earlier. Oak trees in Europe come into leaf days to weeks earlier than they did in the mid-twentieth century.

Problems arise when interacting species shift at different rates, a phenological mismatch. In the Netherlands, the peak abundance of caterpillars moved earlier, but pied flycatchers, which migrate from Africa, did not advance their arrival as much. Their chicks hatched after the caterpillar peak, and flycatcher populations declined most in the areas where the mismatch was greatest.

3. Physiology and body size

Many organisms are affected directly by heat and by changes in body chemistry.

Sea turtles have temperature-dependent sex determination: warmer sand produces more females. On some beaches of the northern Great Barrier Reef, almost all young green turtles hatched in recent decades are female.
Ocean acidification, caused by the ocean absorbing CO₂, makes it harder for corals, oysters, sea snails and some plankton to build shells and skeletons.
Some animals are becoming smaller as temperatures rise, consistent with Bergmann’s rule; studies of North American migratory birds have documented gradual decreases in body size over recent decades.

4. Extreme events

Climate change is making heatwaves, droughts, wildfires, floods and marine heatwaves more frequent and intense. In 2019–20, Australia’s Black Summer fires affected an estimated 3 billion animals. A 2011 marine heatwave off Western Australia wiped out large areas of kelp forest. Heat and drought have caused mass die-offs of flying foxes, and in 2021 a heat dome over the Pacific Northwest killed an estimated billion or more intertidal animals.

Ecosystems at the front line

Coral reefs. When water temperatures stay about 1 °C above the normal summer maximum for several weeks, corals expel their symbiotic algae and bleach. Bleached corals can recover if temperatures fall, but repeated or prolonged bleaching kills them. The world experienced its fourth global bleaching event in 2023–2025, affecting reefs in more than 80 countries.
Sea ice. Arctic summer sea ice has declined by roughly 13 percent per decade since satellite records began in 1979, reducing the hunting platform polar bears depend on. In Antarctica, record low sea ice in 2023 caused breeding failures in emperor penguin colonies.
Mountains. Glaciers are retreating worldwide, and alpine species have limited space to move.
Cloud forests and islands, where species have small ranges and nowhere to go.
The Amazon, where warming, drought and deforestation together raise the risk that parts of the forest could turn into degraded savanna.

How species respond

Species facing climate change have three broad options:

Move to track suitable climate, if habitat is connected.
Adjust through behavioural or physiological flexibility (plasticity), for example by breeding earlier or seeking shade.
Adapt genetically over generations, which is possible for species with short generation times and large, diverse populations.

Species that cannot do any of these fast enough face local or global extinction. Those most at risk typically have narrow tolerances, small ranges, specialised diets, long generation times and limited ability to disperse.

What conservation can do

Cut emissions. Limiting warming is the single most important action. Every fraction of a degree matters: coral reefs are projected to decline by 70 to 90 percent at 1.5 °C of warming and by over 99 percent at 2 °C.
Protect and connect habitat so species can move, through corridors, protected area networks and “climate refugia” that stay cooler.
Reduce other pressures such as habitat loss, overfishing and pollution, which make species more resilient to climate stress.
Nature-based solutions. Protecting and restoring forests, peatlands, mangroves and seagrass stores carbon and benefits wildlife at the same time.
Assisted migration and breeding for heat tolerance, controversial but increasingly discussed for species with nowhere to go, including corals.

Case study: the pika

The American pika is a small relative of rabbits that lives on rocky slopes in the mountains of western North America. Pikas have thick fur and a high body temperature, and can die after a few hours at temperatures above about 25 °C. In the Great Basin, pikas have disappeared from many lower-elevation sites where they were recorded in the twentieth century, and surviving populations have moved upslope. The pika has become a symbol of how climate change pushes mountain species towards the summits.

Try it yourself

Look up the long-term average temperature for your region and compare it with the last ten years. Then find one species in your area whose timing or range might be affected. Citizen science projects such as Nature’s Calendar, iNaturalist and eBird collect exactly this kind of data.

Common misconceptions

“The climate has always changed, so this is natural.” Past changes were usually far slower. The current warming is driven mainly by human greenhouse gas emissions and is happening in decades, not millennia.
“A degree or two is too small to matter.” Global averages hide much larger regional and seasonal changes, and many species live close to their thermal limits.
“Species will just move.” Movement requires connected habitat, and some species, such as those on mountaintops or islands, have nowhere to go.

Key terms

Greenhouse effect: warming caused by gases that absorb heat radiated from Earth’s surface.
Range shift: a change in where a species lives.
Phenology: the timing of seasonal biological events.
Phenological mismatch: loss of synchrony between interacting species.
Coral bleaching: loss of symbiotic algae by stressed corals.
Climate refugium: an area where climate change is slower or milder.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Climate Change Ecology