Evidence for climate change and its causesEdexcel International A Level Biology: Revision notes

Carbon dioxide records come from two sources. Direct atmospheric measurements (for example at Mauna Loa since 1958) show a steady rise, from about 315 ppm to over 420 ppm, with a small seasonal oscillation caused by photosynthesis in the northern hemisphere. For earlier times, scientists analyse the air trapped in bubbles in ice cores. Each year's snow is compacted into a layer, so deeper ice is older, and the gas in the bubbles gives the carbon dioxide concentration at the time. The ice cores show about 280 ppm before the Industrial Revolution, and concentrations that rose and fell with temperature over hundreds of thousands of years.

Temperature records come from thermometers, ships, weather stations, ocean buoys and satellites. Averaged over the globe, they show a rise of about 1.1 °C since 1850, with most of it in recent decades. Because the records come from many independent sources and agree, scientists have confidence in them.

Peat bogs are waterlogged, acidic and anaerobic, so decomposers are inhibited and plant remains are preserved. Pollen grains have a tough outer wall (exine) with a shape and surface pattern that is characteristic of the species, so grains can be identified under a microscope.

A core is taken from the bog and samples from each depth are examined. Deeper peat is older, and layers can be dated, for example by radiocarbon dating. The proportions of pollen from different species show which plants were growing at the time. Because different plants are adapted to different temperatures, this gives an indication of past climate: a layer rich in pollen from cold-tolerant plants suggests a cold climate, while oak and hazel suggest a warmer one.

Limitations: pollen may be blown in from far away, plants differ in how much pollen they produce, and vegetation is also affected by soil, grazing and human activity, so pollen gives an indirect, approximate indication of climate.

Dendrochronology is the study of annual rings in trees. Each year the cambium adds a ring of xylem. In a warm year with plenty of water the growing season is long and productive, so the ring is wide. In a cold or dry year the ring is narrow.

A record longer than the life of one tree is built by cross-dating: the pattern of wide and narrow rings in a living tree is matched with the pattern in the outer rings of an older piece of wood (a beam or a preserved trunk), and so on back in time. Counting rings back from the outer ring of a living tree (or from the known felling date) gives each ring a calendar year.

Limitations: ring width is affected by many factors besides temperature, including rainfall, carbon dioxide concentration, soil nutrients, competition from other trees and disease. Data from one species or site may not represent the wider climate, so results are checked against other evidence.

A correlation is a relationship between two variables: they change together. A causal relationship means that a change in one variable directly brings about the change in the other. A correlation does not by itself prove causation, because both variables may be affected by a third factor, or the relationship may be by chance, or the direction of the cause may be unclear.

To show causation, scientists look for a plausible mechanism (here, carbon dioxide absorbing infrared radiation), for consistency across independent data sets, for the cause to come before the effect, and for other explanations to be ruled out. A statistical test, such as a correlation coefficient, shows how strong a relationship is and whether it is likely to be due to chance, but it still cannot show a mechanism.

Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth's surface. The surface emits longer-wavelength infrared radiation. Greenhouse gases (carbon dioxide, methane, water vapour, nitrous oxide) absorb some of this infrared and re-radiate it in all directions, including back to the surface, so the lower atmosphere is warmer than it would otherwise be. This greenhouse effect is natural and makes life possible.

The enhanced greenhouse effect is caused by human (anthropogenic) activity that increases greenhouse gas concentrations: burning fossil fuels (carbon dioxide), deforestation (less photosynthesis, carbon dioxide from burning and decay), cattle and rice paddies (methane) and fertilisers (nitrous oxide). More infrared is trapped, so the Earth warms.

In the carbon cycle, carbon dioxide is removed from the atmosphere by photosynthesis (and dissolves in oceans) and is returned by respiration, decomposition and combustion. Carbon is stored for long periods in forests, peat, oceans and fossil fuels. Burning fossil fuels moves carbon from a long-term store into the atmosphere much faster than natural processes remove it.

Knowledge of the cycle suggests how to reduce atmospheric carbon dioxide: use less fossil fuel (renewable energy, efficiency) so less carbon is released; increase uptake by photosynthesis (afforestation and reforestation); protect carbon stores such as forests and peat bogs, since draining or burning them releases carbon dioxide; and capture carbon dioxide from industrial emissions and store it.