Isn’t the climate always changing?

Yes. There is natural variability in Earth’s climate but the current climate change is very unusual as it is not exclusively part of a natural cycle.

Natural factors include volcanic eruptions, aerosols and phenomena such as El Niño and La Niña (which cause warming and cooling of the Pacific Ocean surface). Natural climate variations can lead to periods with little or no warming, both globally and regionally, and other periods with very rapid warming. However, there is an underlying trend of warming that is almost certainly caused by man’s activities.

Climate scientist Jim Haywood explains more about aerosols and climate change

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Aren’t all these changes down to the Sun and natural factors?

No. Many factors contribute to climate change. Only when all the factors are considered can we explain the size and patterns of climate change over the last century.

Although some people claim that the Sun and cosmic rays are responsible for climate change, measured solar activity shows no significant change in the last few decades, while global temperatures have increased significantly. Since the Industrial Revolution, additional greenhouse gases have had about ten times the effect on climate as changes in the Sun’s output.

Much of the relatively small climate variability over the last 1,000 years, but before industrialisation, can be explained by changes in solar output and occasional cooling due to major volcanic eruptions. Since industrialisation, CO₂ has increased significantly. We now know that man-made CO₂ is the likely cause of most of the warming over the last 50 years.

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Do climate scientists really agree about climate change?

Yes. The overwhelming majority of climate scientists agree on the fundamentals of climate change — that climate change is happening and has recently been caused by increased greenhouse gases from human activities.

The core climate science from the Intergovernmental Panel on Climate Change (IPCC) was written by 152 scientists from more than 30 countries and reviewed by more than 600 experts. It concluded that most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in man-made greenhouse gas concentrations.

Find out more about the IPCC report

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Are computer models reliable?

Yes. Computer models are an essential tool in understanding how the climate will respond to changes in greenhouse gas concentrations, and other external effects, such as solar output and volcanoes.

Computer models are the only reliable way to predict changes in climate. Their reliability is tested by seeing if they are able to reproduce the past climate, which gives scientists confidence that they can also predict the future.

But computer models cannot predict the future exactly. They depend, for example, on assumptions made about the levels of future greenhouse gas emissions.

What can climate scientists tell us about the future?

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Surely, the impact of human activity is small?

No. Greenhouse gases are produced naturally and commercially. Both types influence climate change.

All the greenhouse gases combined (the main ones being water vapour, CO₂, methane and nitrous oxide) are only a tiny part of the atmosphere, making up less than 0.5%. Yet it is scientifically proven that these gases trap heat, keeping the planet 30 °C warmer than it would be otherwise and able to sustain life. Any changes in the levels of these gases, such as those recently brought about by human activity, will have a significant effect on global temperatures.

Keeping the climate stable is important for the well-being of the Earth. But there is now very strong evidence that man-made greenhouse gases are causing climate change.

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Are you sure there’s a link between temperature rise and CO₂

Yes. Temperature and CO₂ are linked. Studies of polar-ice layers show that in the past, rises in temperature have been followed by an increase in CO₂. Now, it is a rise in CO₂ that is causing the temperature to rise.

Concentrations of CO₂ have increased by more than 35% since industrialisation began, and they are now at their highest for at least 800,000 years.

When natural factors alone are considered, computer models do not reproduce the climate warming we have observed. Only when man-made greenhouse gases are included do they accurately recreate what has happened in the real world.

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Isn’t the recent warming due to the growth of our towns and cities?

No. The climate is warming everywhere because of CO₂ emissions. Temperatures in cities are unnaturally high because of the warmth from heating homes and offices, heavy traffic, high concentrations of people and heat stored in buildings and concrete.

Our observations come from urban and rural areas on land and from the sea, which covers 70% of the Earth. We manage data from cities carefully to ensure they do not skew our understanding of climate change.

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How does El Niño affect our climate?

El Niños are natural variations in climate. When there is an El Niño the tropical eastern Pacific is warmer than average and global temperatures are also warmer. A particularly strong El Niño occurred in 1998 — the warmest year on record across the globe.

The opposite effect is La Niña. When La Niña occurs, it’s cold in the eastern Pacific, resulting in cooler than average temperatures. 2007 and 2008 saw a long-lasting La Niña, but 2008 was still the tenth warmest in the global record.

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Has global warming now stopped?

No. The rise in global surface temperature has averaged more than 0.15 °C per decade since the mid-1970s. The 10 warmest years on record have occurred since 1997. Global warming does not mean that each year will necessarily be warmer than the last because of natural variability, but the long-term trend is for rising temperatures. The warmth of the last half century is unprecedented in, at least, the previous 1,300 years.

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