Climate change: An introduction

Climate change is the current rapid warming of the Earth's climate caused by human activity. If left unchecked (and current responses are doing little to halt it) it poses an unprecedented threat to human civilisation and the ecosystems on this planet.

What does it mean to say the climate is changing?

First, 'climate' is very different from 'weather'. Weather changes by the hour and, especially in the UK, naturally varies widely between years. We know the climate is changing because, averaged out over longer periods, the global mean temperature has been consistently rising, across land and sea. It is now about 0.8C above pre-industrial times.

The below graph shows global temperatures from 1860 to 2015. The data used came from the National Oceanic  and Atmospheric Administration (NOAA). For more information, click here.

Climate Lab Book created an animated climate spiral, illustrating the increase in global temperatures from 1850 to the present.

The world has been experiencing changes in climates, affecting millions of lives. Already, there has been the bleaching of coral reefs, the sea ice volume in the Arctic has been reaching new lows, an increase in the number of natural disasters worldwide (such as wildifres, droughts, floods) and the mass migration of species. For more information, you can read more about the current effects of climate change here.

What is the greenhouse effect?

Certain gases in the Earth's atmosphere (water vapour, CO2, methane and others) allow sunlight to pass through, but then stop the heat from escaping back out into space - much like glass in a greenhouse. Without this, our planet would be uninhabitable to most forms of life. However, by changing the balance of gases in the atmosphere, humans have increased the greenhouse effect, causing the rising temperatures we now see.

Where do greenhouse gases come from?

As explained above, these gases exist naturally in our atmosphere. The most significant increases are in carbon dioxide (there is now over a third more CO2 in our atmosphere than there was before the industrial revolution) and methane. Methane is a more potent greenhouse gas, but it only remains in the atmosphere for about a decade. Carbon dioxide lasts for about 100 years or more, so even if we stopped emissions from human activities altogether, the planet would continue to warm up from the gases already emitted. The main causes of increased CO2 in the atmosphere are burning fossil fuels (coal, oil and gas), and deforestation and other changes in land use that release stored CO2 and methane.

The below graph, also known as the Keeling Curve, shows CO2 levels today and how this compares with the last 10,000 years.

Is there any doubt about what's happening?

The idea of an urgent shift away from fossil fuels is not welcome to everyone, and those who seek to delay or prevent this have been very successful in spreading the idea that climate scientists are uncertain about climate change (or even fraudulent!). Unfortunately there is, as legal terminology has it, no 'reasonable doubt' about climate change.

Could the rise in atmospheric carbon be coming from somewhere else?

Humans are currently emitting around 30 billion tonnes of CO2 into the atmosphere every year. Of course, it could be coincidence that CO2 levels are rising so sharply at the same time so let's look at more evidence that we're responsible for the rise in CO2 levels:

  • When we measure the type of carbon accumulating in the atmosphere, we observe more of the type of carbon that comes from fossil fuels
  • This is corroborated by measurements of oxygen in the atmosphere. Oxygen levels are falling in line with the amount of carbon dioxide rising, just as you'd expect from fossil fuel burning which takes oxygen out of the air to create carbon dioxide
  • Further independent evidence that humans are raising CO2 levels comes from measurements of carbon found in coral records going back several centuries. These find a recent sharp rise in the type of carbon that comes from fossil fuels

How do we know that the extra CO2 in the atmosphere is warming the planet through the greenhouse effect?

  • CO2 absorbs heat at particular wavelengths. Satellites measure less heat escaping out to space, at the particular wavelengths that CO2 absorbs heat, while surface measurements show more heat returning at CO2 wavelengths.
  • If an increased greenhouse effect is causing global warming, we should see certain patterns in the warming. For example, the planet should warm faster at night than during the day. This is indeed being observed.
  • Another expected result of greenhouse warming is cooling in the upper atmosphere, otherwise known as the stratosphere. This is exactly what's happening.
  • With the lower atmosphere (the troposphere) warming and the upper atmosphere (the stratosphere) cooling, another consequence is the boundary between the two layers should rise as a consequence of greenhouse warming. This has also been observed.
  • An even higher layer of the atmosphere, the ionosphere, is expected to cool and contract in response to greenhouse warming. This has been observed by satellites.

(The above Q&A was taken from Skeptical Science, where you can read more about the evidence and find the answers to lots more questions like "Could the sun be causing it?" and "What about the Mediaeval warm period?")

What can we expect to happen next?

That depends on what we do now. Because of all the greenhouse gases already in the atmosphere, if the human race died out tomorrow, we'd still expect the planet to continue heating up. If we carry on emitting at the rate we are today, it will heat up much more rapidly. Rather than just warming, it makes more sense to think of it as the climate becoming more unstable, with extra energy in the system. Extreme weather events will become more common, ecosystems will be put under stress and so will human agriculture and water supplies. Some parts of the world are particularly vulnerable, such as sub-Saharan Africa, but no area will be immune.

The pledges that governments have made so far to cut emissions are insufficient. Even if implemented fully, they are consistent with an average global temperature rise of 4C (see, e.g. the IEA). However, there are now concerns that global temperatures could rise at a greater rate due to the Earth's climate sensitivity being non-linear. A rise of 2C has been viewed as a 'safe limit' in international negotiations, but this does not fully take into account either the serious humanitarian and ecosystem impacts of this temperature rise in many parts of the world. The poorest countries of the world and small island states face threats, for the latter to their actual existence, with any global warming above 1.5°C. Nor does it consider the risk of triggering positive feedback mechanisms. An example of the latter is the release of frozen carbon and methane from melting in the polar regions, which would further accelerate warming. Since there is in reality no clear 'safe' zone, this demands an even more urgent response to cutting emissions.

What would a world 4C hotter look like?

  • Increases of 6°C or more in average monthly summer temperatures would be expected in large regions of the world, including the Mediterranean, North Africa, the Middle East, and parts of the United States, with heatwaves raising temperatures further.
  • Sea levels would rise by 0.5 to 1 metre at least by 2100, and by several metres more in the coming centuries. Major cities would be threatened by flooding.
  • As oceans absorb excess CO2 they would become around 2 1/2 times as acid as they are now, and marine ecosystems would be devastated by this on top of the impacts of warming, overfishing and habitat destruction. Most coral reefs would be long destroyed (from around 1.4C temp rise)
  • As ecosystems undergo rapid transition, mass extinctions are likely.
  • Agriculture would be under extreme stress in much of the world, especially the poorest regions.

Read more

There is a vast amount of information on the internet about the science of climate change, from the simple to the deeply technical, and some which is just plain wrong (find out more about climate sceptics). For example, here is a brief introduction to climate science and further discussion of the climate threat.

'Climate Emergency', written by the campaign's former National Coordinator, Phil Thornhill, is a good introduction to important concepts in the science of climate change.

For an explanation of where we are heading, look at the presentation 'Climate Change: Going Beyond Dangerous' by Professor Kevin Anderson.

More on the impacts of climate change from the World Bank: 'Turn Down the Heat: Why a 4°C Warmer World Must be Avoided'

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