When snow falls to the ground it traps tiny amounts of air between the snow flakes. As more snow falls, the weight above compresses the snow below into ice, trapping these tiny pockets of air. These form a record of the air composition at the time the snow fell. The deeper down the air pocket, the older the air.

Scientists have drilled into the arctic snow, down through hundreds of metres of ice, extracting ice cores and thus air pockets from thousands of years ago. By analysing the gases in these and how far down they are in the ice core, they can work out how much carbon dioxide and other gases were present in the air when the snow fell and how old it is.

In some parts of the world’s oceans, the sea floor is made of mud hundreds of metres thick. Tiny sea creatures have lived and died over vast numbers of years, falling to the sea floor to become mud. By extracting cores of mud at increasing depths, scientists can in effect look back through time at the remains of ancient sea creatues.

Sea shells = CO₂

Some tiny molluscs grew shells, which still be seen in the mud. Since they build shells by combining calcium in the sea water with carbon dioxide, the thickness of their shells gives a good indication of how much CO₂ was present in the water when they lived. Plotting the thickness of their shells through the depth of mud gives a pattern that shows changes in CO₂ in the atmosphere over thousands of years.

Pollen = temperature

Also buried in the mud are grains of pollen from plants that were growing nearby at the time the mud was formed. Because different plants grow at specific temperatures, the presence of pollen from specific species is evidence of the local temperature when the pollen was deposited.

Since the 1950s satellites have recorded temperature data and taken photographs from across the earth. Climate change scientists now have access to data covering over five decades.

Many glaciers around the world are melting at an alarming rate. Look at this website documenting recent and old photos of the same glaciers.