Carbon dioxide and methane as greenhouse gases

Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases.

The greenhouse effect is not itself a problem — without it the Earth would be far too cold for life. What is at issue is the increase in the amounts of these gases.

You should be able to describe the greenhouse effect in terms of the interaction of short and long wavelength radiation with matter. The sequence, in the order that scores:

1. Short wavelength radiation from the Sun passes through the atmosphere. 2. It is absorbed by the Earth's surface, which warms up. 3. The warmed Earth emits radiation of a longer wavelength. 4. Greenhouse gases in the atmosphere absorb that long wavelength radiation. 5. They re-emit it in all directions, including back towards the Earth, so energy is kept in the atmosphere and the temperature rises.

Some human activities increase the amounts of greenhouse gases in the atmosphere. These include carbon dioxide and methane.

You should be able to recall two human activities that increase the amounts of each of these gases.

Based on peer-reviewed evidence, many scientists believe that human activities will cause the temperature of the Earth's atmosphere to increase at the surface, and that this will result in global climate change.

However, it is difficult to model such complex systems as global climate change. This leads to simplified models, speculation and opinions presented in the media that may be based on only parts of the evidence, and which may be biased.

You should be able to evaluate the quality of evidence in a report about global climate change given appropriate information, describe uncertainties in the evidence base, and recognise the importance of peer review of results and of communicating results to a wide range of audiences.

Questions to ask of a report: who produced it, and might they gain from a particular conclusion? Has it been peer reviewed — checked by other scientists working in the field — or is it an opinion piece? Does it use all the evidence, or only the part that supports its conclusion? Is a range or an uncertainty given, or only a single figure?

The uncertainties themselves are worth naming: the climate is an extremely complex system with many interacting variables, so models have to be simplified; and predictions depend on assumptions about future human behaviour that nobody can be certain of.

Peer review matters because it lets other scientists check the method and the data before the work is accepted, which catches errors and bias. Communicating results to a wide range of audiences matters because decisions about climate are made by governments and by the public, not only by scientists.

An increase in average global temperature is a major cause of climate change. There are several potential effects of global climate change.

You should be able to describe briefly four potential effects of global climate change, and discuss the scale, risk and environmental implications of global climate change.

Discussing scale, risk and environmental implications means going beyond the list. Scale: these are global changes, affecting every country, not local ones. Risk: the effects are not certain, and both their size and their timing are hard to predict, but the consequences if they do occur are severe and largely irreversible. Environmental implications: whole ecosystems, not just individual species, are affected — and the countries least able to adapt are often the ones most exposed.

The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.

The carbon footprint can be reduced by reducing emissions of carbon dioxide and methane.

Note the phrase "full life cycle": it counts the emissions from obtaining the raw materials, manufacture, transport and use, right through to disposal — not just the emissions while the product is being used.

You should be able to describe actions to reduce emissions of carbon dioxide and methane, and give reasons why actions may be limited.

The reasons actions are limited fall into a few groups, and naming the group is usually what the mark is for: cost, incomplete scientific or technical development, disagreement between governments, lack of public information or willingness to change lifestyles, and the fact that emissions in one country affect everyone, so no country can solve it alone.