What is the difference between weather and climate?

To understand climate change, it’s important to recognise the difference between weather and climate. Weather is the temperature, precipitation (rain, hail, sleet and snow) and wind, which change hour by hour and day by day. Climate is the average weather and the nature of its variations that we experience over time.

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What is the greenhouse effect?

The greenhouse effect is the natural process of the atmosphere letting in some of the energy we receive from the Sun (ultraviolet and visible light) and stopping it being transmitted back out into space (infrared radiation or heat). This makes the Earth warm enough for life.

For several thousands of years the atmosphere has been delicately balanced, with levels of greenhouse gases relatively stable. Human influence has now upset that balance and, as a result, we are seeing climate change.

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How are we causing climate change?

Human activities, like burning coal, oil and gas, have led to an increase in greenhouse gases in the atmosphere, causing an enhanced greenhouse effect and extra warming.

As a result, over the past century there has been an underlying increase in average temperatures which is continuing. Globally, the ten hottest years on record have all been since 1997.

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What will happen if we don’t act to reduce emissions?

If emissions continue to grow at present rates, CO₂ concentration in the atmosphere is likely to reach twice pre-industrial levels by around 2050. Unless we limit emissions, global temperature could rise as much as 7 °C avove pre-industrial temperature by the end of the century and push many of the world’s great ecosystems (such as coral reefs and rainforests) to irreversible decline.

Even if global temperatures rise by only 2 °C, 20–30% of species could face extinction. We can expect to see serious effects on our environment, food and water supplies, and health.

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Which gases are causing the most change?

The main greenhouse gas responsible for recent climate change is CO₂. This has been released in huge quantities by our modern way of life. Levels have also increased due to the destruction of rainforests, which play an important role in absorbing CO₂.

Human activities are increasing other greenhouse gases too, such as methane and nitrous oxide. Methane is produced by bacteria that live in places like landfill sites, peat bogs and in the guts of animals like cows and sheep. Nitrous oxide is increased by the use of nitrogen fertiliser in agriculture.

Both these gases have a powerful greenhouse effect and also contribute to climate change. However, they have not been released in such large quantities as CO₂ and methane does not last for as long in the atmosphere. So, while they make a significant contribution to climate change, it is man-made CO₂ which has by far the greatest influence.

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Will it get hotter everywhere?

Yes. Even if the concentrations of greenhouse gases and aerosols stabilised at the year 2000 levels then we would still expect temperatures to reach 1.4 °C above pre-industrial levels by 2100 (Source: IPCC).

Even if emissions peak in 2015 and decrease rapidly at around 3% every year after that, there may only be a 50:50 chance of keeping global temperature rise below 2 °C. Every delay of ten years in the peak emissions would add about 0.5 °C of warming.

Map showing how the world will warm by early, mid, late 21st century for a medium–high emissions scenario (courtesy of IPCC)

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Which areas are warming the most?

In recent decades the Arctic has been heating twice as fast as the rest of the world, largely because Arctic ice, which reflects sunlight and keeps the surface cool, has decreased. In particular, summer Arctic sea-ice has shrunk by about 10% per decade since 1979. Land-ice and snow-cover have also decreased — a bigger effect in the short-term because land heats up more quickly than the sea.

The Northern Hemisphere is warming more than the Southern Hemisphere. This is because the Northern Hemisphere has more land mass, which heats faster than water.

(courtesy of IPCC)

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Why are sea- levels rising?

A warming climate raises sea-levels in two ways:

1. Thermal expansion — as water warms it expands, like liquid in a thermometer. A warming climate will heat oceans, causing sea-levels to rise.
2. Ice-melt — large amounts of water are locked in glaciers, permafrost and ice-caps around the world. Warmer weather is causing these to melt. Water from land-based ice will flow into the oceans, raising sea-levels. Sea-levels around the UK have already risen by 10 cm since 1900 and scientists are still researching how quickly they will continue to rise.

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