Causes of Climate Change

The greenhouse effect is not the problem. It is the reason the planet is warm enough to live on at all. The problem is that we have made it stronger, and this page is about exactly how – which gases, which human activities, and why the warming then starts to speed itself up.

The greenhouse effect, in order

Start with the sequence, because most marks in this topic are for getting the steps in the right order.

Radiation from the Sun reaches the Earth and passes through the atmosphere. It hits the surface, and the surface absorbs that energy and then radiates it back out. The energy that leaves the ground is not the same as the energy that arrived: it comes back off at longer wavelengths, as infrared radiation, which we experience as heat.

That change of wavelength is the whole trick. Certain gases in the atmosphere let the incoming sunlight straight through, but absorb the longer-wavelength radiation coming back up. Having absorbed it, they re-emit it in all directions, so a good share of it heads back down towards the ground instead of out into space.

Where the extra carbon dioxide comes from

Atmospheric carbon dioxide has gone up and down throughout Earth’s history on its own. Volcanic eruptions push it up; the weathering of limestone rocks pulls it back down. Those are natural fluctuations and they have been happening for millions of years.

What is different now is the size and speed of the change. Since the industrial revolution – which began in the late 1700s, when burning fossil fuels to run factories, transport and homes became normal – carbon dioxide levels have climbed to the highest they have ever been in Earth’s history. Burning any fossil fuel releases carbon dioxide.

Fossil fuel combustion is not the only route, though. Carbon dioxide is also released when natural stores of carbon are damaged or destroyed by human activity. Those stores are called carbon sinks.

Where the extra methane comes from

Methane (CH₄) is a simple hydrocarbon. It sits in the atmosphere as a gas, it sits underground, and it is the main component of the fossil fuel we call natural gas. Some methane is produced by naturally occurring processes in certain bacteria, and always has been – but levels have climbed sharply over the last 150 years or so, and human activity is behind that climb.

• Ruminant livestock. Methane is produced in the guts of ruminant mammals such as cattle, and released. Intensive farming has hugely increased the number of these animals, and so their contribution.
• Landfill sites. Organic matter such as food waste decomposes in landfill and releases methane.
• Fossil fuel extraction. Getting fossil fuels out of the ground releases methane that was trapped there.
• Rice paddy fields. Anaerobic bacteria in the waterlogged soil release methane.

And there is one more, which matters because it is not a choice anyone makes: as global warming heats the poles, methane is released from natural stores such as permafrost – ground that stays frozen all year round. Warming releases methane, and methane causes more warming. Hold onto that, because it comes back later on this page.

NOS: correlation is not causation

Correlation analysis means measuring two variables and looking at whether there is a relationship between them. There are two kinds you need to name:

Climate change research is full of correlations. Evidence from Antarctic ice cores shows a positive correlation between global temperatures and atmospheric carbon dioxide stretching back hundreds of thousands of years – when one is high, so is the other.

Here is the part examiners are actually testing. A correlation shows an association between two variables. On its own it does not show causation – that a change in one has caused the change in the other. Two reasons why not:

• You cannot tell which variable influenced which. Did the carbon dioxide raise the temperature, or did the temperature raise the carbon dioxide?
• There may be an unknown third variable driving both of them.

So how do scientists get from correlation to a causal claim? By adding other evidence. In this case we already understand the physics of how greenhouse gases absorb infrared radiation, and that mechanism, combined with the correlation and with other lines of evidence, builds a strong case. The correlation is one strand of the argument, not the whole rope.

Positive feedback: why warming speeds up

In everyday English “positive” means good. In biology it means something quite different, and here it means something rather alarming.

Positive feedback is destabilising. It amplifies a deviation instead of correcting it, and it can drive a system towards a tipping point – the moment when the system stops drifting gradually and suddenly shifts into a new, different state.

Global warming has a positive feedback effect on the Earth and its atmosphere. Put bluntly: global warming causes more global warming. Several separate mechanisms do this, and the clearest one to draw is the loss of ice.

Loss of reflective snow and ice

How much light a surface reflects is called its albedo. High albedo means a lot of light is reflected; low albedo means most of it is absorbed. Snow and ice are pale, so they have a high albedo. Rock, soil and open ocean are dark, so their albedo is low.

As the polar ice caps melt, the pale surface is replaced by dark surface. The Earth’s overall albedo falls, more of the Sun’s energy is absorbed rather than bounced back, and the planet warms further – which melts more ice.

The other three feedbacks

Ice is the easiest one to picture, but three more turn up in exam questions, and all of them end the same way – with more greenhouse gas in the atmosphere.

Note the detail on permafrost, because it is worth a mark on its own: permafrost is a huge carbon sink precisely because the organic material inside it cannot decompose at low temperatures. Decay organisms are inactive when it is cold. Warm it up and you have not just added heat – you have switched on a store of carbon that had been safely locked away.

Worked examples