What is climate change and why does it matter?

Find out what climate change is, why it matters and what it could mean for our collective future.

What are weather, climate and climate change?

Weather refers to atmospheric conditions, such as rain or snow, happening in a place at a specific moment in time.

Climate is how much, on average, a type of weather will occur over a longer period.

Climate scientist Professor Joeri Rogelj is Director of Research at the Grantham Institute and Professor of Climate Science & Policy at the Centre for Environmental Policy at Imperial College London. He’s contributed to and led several major climate change assessments.

“Climate change is how the characteristics of the weather we experience in a certain place change,” Joeri explains. “It can get hotter or wetter on average or have more concentrated rain in a short period, but then get longer dry periods. All of that can be a result of climate change.”

‘Global warming’ is often used interchangeably with the term climate change. However, Joeri notes that it’s preferable to use the latter because the warming atmosphere and oceans are just some of the effects we’re seeing.

“It's not just about temperature. Places are also becoming wetter or drier, and in some the seasons are moving. Most importantly, in a few regions and seasons, it may actually at times be cooler than we’re used to. That’s confusing if you just talk about global warming.”

What causes climate change?

The main driver of current climate change is the emission of greenhouse gases, most importantly carbon dioxide and methane.

These gases are primarily released when fossil fuels – such as coal, oil and natural gas – are burnt. Meat and dairy production, producing cement and some industrial processes, such as the production and use of fertilisers, also emit greenhouse gases.

Greenhouse gases trap heat in our atmosphere, Joeri explains. “Energy from the Sun falls on our planet and normally gets reflected back as infrared radiation. But instead of escaping back out into space, this radiation gets absorbed by molecules of greenhouse gases, which then emit them in all directions.”

This process causes more heat to be kept near Earth’s surface, warming our world.

Since the mid-nineteenth century, the world has emitted more than 2.2 trillion tonnes of carbon dioxide. Carbon dioxide molecules hang around in the atmosphere for hundreds of years. Methane is more than 25 times as potent as carbon dioxide and is responsible for around 0.5°C of total warming, although its molecules have a shorter ‘atmospheric lifetime’ of around 12 years.

How do we know climate is changing?

There are measuring stations all around the world that keep track of air and sea temperature. From these measurements it’s clear that temperatures are rising.

“There are many more indicators that tell us that the Earth is warming. For example, on a warming planet we would expect polar ice caps and glaciers to melt. It’s clearly observed that those are melting,” explains Joeri.

We know that greenhouse gases are causing change. Thanks to studies that look at how carbon dioxide absorbs infrared radiation, for example, there’s a scientific understanding of how the planet warms as a result of emissions. This allows climate scientists to discount the theory that global warming is being caused by an increase in the Sun’s intensity, for example.

We also know that greenhouse gases are primarily emitted by fossil fuel combustion.

“To burn carbon and produce carbon dioxide, you need oxygen. The amount of oxygen that’s in the atmosphere is reducing at exactly the right amount for the increase in carbon dioxide to be caused by combusting fossil fuels,” explains Joeri.

We can also see evidence in the ratios of different types of carbon. Fossil fuels are, essentially, ancient plants. Plants now and in the past preferentially take up carbon-12. In normal conditions, the ratio between carbon-12 and carbon-13 is constant.

“What we can see is that the ratio of carbon-13 in our atmosphere is going down at exactly the rate you would predict if the carbon dioxide increase was due to burning fossil fuels.”

Effects of climate change

Climate change doesn’t have the same effects everywhere. The planet is generally getting hotter, but some regions and seasons can at times be temporarily cooler. Some places will see drawn-out seasons, while others may experience concentrated bursts of extreme weather.

Extreme weather events, such as hurricanes, floods, heatwaves, drought and wildfires, are predicted to become more intense and frequent.

“Pretty much any weather event is influenced by climate change. As scientists we can estimate how much climate change has made a certain event more likely or more intense than it would have been without climate change,” explains Joeri.

When the world warms, ice melts. Arctic sea ice could disappear entirely in a warming world, and Greenland and Antarctica’s ice sheets could be destabilised. This would result in large sections melting, which would add more liquid to the ocean.

Ice also reflects the Sun’s energy. Without ice, more heat is absorbed by the ocean. Water expands as it warms – this is called thermal expansion. This effect means that the ocean takes up more space, causing sea levels to rise. Even with rapid emission cuts, sea levels are expected to rise by around 26 to 53 centimetres by 2100.

Along with melting ice sheets and glaciers, rising global temperatures could cause rainforests to die and widespread species extinctions.

How does climate change affect us?

How climate change will affect you depends on who you are and where you live.

Around 190 million people currently live in areas that, due to rising sea levels, are expected to be under high tide levels by 2100. This could cause a massive displacement of populations. Low-lying island nations, such as Tuvalu and the Maldives, are incredibly vulnerable to sea level rise and could be lost to the sea.

Hundreds of millions of people rely on seafood as their main source of protein. Warming and more acidic waters could destroy marine food chains by affecting their base, such as krill or coral reefs.

Longer-lasting droughts could devastate crops, threating food security. Reservoirs drying up, as well as the loss of glaciers, could make drinking water scarce.

Increased precipitation can cause deadly flooding, as well as lowering indoor air quality. This could affect our health as dampness benefits moulds and fungi.

More than four billion people currently live in urban areas. By 2050 this will have risen to an estimated 6.7 billion. City dwellers aren’t exempt from climate change’s effects. Urban populations usually rely on rural areas for inputs such as food and water. If climate change disrupts these important connections, it could heavily affect those in urban areas.

Natural disasters impact poor and vulnerable populations disproportionately hard and clearly expose the consequences of ignoring social inequalities. With extreme weather increasing, these populations face a heightened level of risk. For example, the urban heat island effect amplifies the effects of temperature extremes in cities. Those unable to afford to buy and run air conditioning may find their health compromised. For those without access to healthcare, heatwaves could become particularly dangerous.

The hazards of climate change also do disproportionate harm to women and girls.

Climate change can also impact our mental health. A growing number of people are experiencing a phenomenon known as eco-anxiety, which is often defined as a chronic fear of environmental doom. It affects people of all ages, but particularly those experiencing climate impacts first-hand and those who have the most to lose in the face of environmental catastrophe.

Joeri points out that “we don’t know what will happen when, exactly. It’s really hard to anticipate, particularly for populations that are already on the edge every year.”

How does climate change affect biodiversity?

The natural world is delicately balanced and many species lives are carefully synchronised with one another. No species – including ours – is completely independent of all others. A 2019 report confirmed that more than one million animal and plant species are now at risk of extinction as a result of human activities.

According to the Joint Nature Conservation Committee, a body that advises the UK Government, key seasonal events that mark the arrival of spring are now happening roughly nine days earlier than they used to. Differing rates of change could mean that species’ lives are no longer synchronised with those they rely on. This mismatch in timings could be devastating for nature.

Many plants are flowering earlier. Migrating birds arrive earlier, leave later and some even are getting smaller. Butterflies are emerging earlier. Birds and amphibians are laying their eggs earlier in the year. Some species are moving into new areas, such as kelps, which form vital marine habitats.

Insects are one of the most vulnerable groups, with less ability than mammals or birds to escape warmer temperatures. Loss of insects, which are a primary food source for many animals, a key pollinator of plants and whose numbers are already plummeting, could cause the ecosystem to collapse.

In aquatic ecosystems, activities to mitigate the side effects of climate change can have negative effects. For example, while building hard flood defences is good for us, they can be a problem for nature. As sea levels rise, sea walls reduce the space for intertidal ecosystems – those found along coasts that are exposed at low tide and underwater at high tide.

A rising sea could also damage important coastal habitats, such as sand dunes and cliffs.

“The ocean looks homogenous, but it also experiences variations. There are ocean heatwaves, where if a particularly warm mass of water comes to an area like coral reefs, it induces loss and mass dieback,” Joeri adds.

The loss of Arctic sea ice takes away a key habitat from animals including polar bears, seals and walruses. The ice is now declining at a rate of more than 12% per decade.

Climate change is just one of the stressors impacting nature. Sea use, invasive species, pollution and the exploitation of organisms are all factors in the threat to nature. Without drastic changes, it’s expected that there will be devastating changes in biodiversity and ecosystems. For example, it’s predicted that that without action, climate change and habitat loss could cause more than 100 species extinctions in Britain by 2080.

Find out more about the link between climate change and the biodiversity crisis.

Is there any hope for the future?

Climate change has been a known problem for around 30 years. Starting to fix it earlier might have made this daunting task much easier.

Joeri says, “we definitely know how to reduce emissions to a significant degree. We’re seeing more impacts of climate change, but we can also see a heightened interest and concern in the general public.”

“Ultimately, reducing emissions is really an issue of public and political will.”

Over the past decades, scientists have estimated the potential impacts of the planet’s average temperature rising by different amounts.

“Based on that information, governments have come together and decided that they don’t want to exceed a 2°C rise. They want to be well below that and pursue efforts to make it as close as possible to 1.5°C,” Joeri explains. These benchmarks are a fundamental element of the Paris Agreement – an international treaty on climate change, adopted in 2015.

The difference in outcomes between 1.5°C and 2°C are considerable. The impacts are the difference of 70% or 99% of coral reefs dying or a summer free of Arctic sea ice once every 100 or 10 years.

If we limit warming to 1.5ºC, we’d expect to see half of the world’s glaciers melt completely by the year 2100. A more extreme rise to 3ºC of warming would mean losing around 70% of all glaciers, likely also causing numerous alpine species to go extinct.

Scientists have determined how much carbon dioxide can still be emitted before this temperature rise limit will be exceeded. This is called the carbon budget and, according to estimates in 2025, it’s very small.

To try and keep us under the emissions limit, scientists are using engineering and economic models to understand how we can transform society. This could include changes such as increasing the share of energy produced by renewables, changing the way food is produced, changing our diets to have a lower carbon footprint and changing the way we build houses and design cities.

It’s important to remember that moving to net-zero emissions won’t be a perfectly co-ordinated transition. What we need is for some countries to lead and show the world that moving to this new way of living and functioning is both possible and beneficial for other sustainability objectives, such as public health and food security.

On whether we’ll ever see a change for the better, Joeri says he’s hopeful.

“Preferably we would have started 25 or 30 years ago, but I’ll take any year at which we start declining steadily towards net-zero.”