A Brief Guide to Climate Science

A Brief Guide to Climate Science

Part 1:  The Science of Climate Change.

Over the last forty years, scientists have become concerned that human activity, particularly burning fossil fuels, is making our planet warmer. More recently, young people like Greta Thunberg have raised public awareness of these concerns, and governments have started to take actions to reduce the use of fossil fuels. But what is the evidence for global warming? How does the “greenhouse effect” actually work and how does it give rise to climate change? And what does it all mean  for us and for our planet?

If we can answer these questions and understand the science, we may be better able to predict, manage, and live with the effects of climate change.

The evidence for Global Warming:

People have been keeping careful weather records across many countries since the 19th century, so we have fairly good record of temperatures from about 1880 onward. However, early methods of data collection were variable, and even some of the more recent records can be unreliable – for example, data from airport weather stations where concrete & tarmac get hot and raise the local temperature. However, it is possible to make corrections for such variations, so as to give a more accurate historical record like those shown below.

Proxy studies – extending the climate record into the past:

This term refers to the study and interpretation of data from various sources, including tree rings, growth layers in corals, ice cores, and microfossils in ocean-floor sediment, all of which can give information about past climate.

There is good evidence to suggest a clear link between global average temperature and the amount of CO2in the atmosphere over historical time. In addition, much older, deeper, ice cores from Greenland & Antarctica show consistently how past ice ages correspond to low  concentrations (<200 ppm) of CO2 whilst warm interglacial periods (often warmer than today) correpond to high (>250 ppm) CO2. Until the 20th century, the CO2 level remained below 280 ppm; in 2022 it passed 420 ppm and continues to rise.

More detail:  https://www.carbonbrief.org/explainer-how-the-rise-and-fall-of-co2-levels-influenced-the-ice-ages/

So, what is the mechanism that causes global warming? To understand that, we need to think about how the Sun's energy interacts with our planet.

Earth's Energy Balance

For Earth to maintain a steady average temperature and a stable climate, the amount of energy radiated from our planet must balance the energy it receives from the Sun. If more energy is received than is lost, the planet gets hotter; if less, then we get colder, until balance is restored. Put simply, energy in = energy out!

Now, some of the incoming energy from the Sun gets filtered out before it ever reaches the Earth’s surface: in particular, harmful X-rays and most ultra-violet (UV) radiation. This happens because these forms of radiation are absorbed by molecules of nitrogen, oxygen and ozone in the upper atmosphere, and converted to thermal (heat) energy. 

(Note: you might wish to research the story of the “ozone hole” discovered in the 1970s).

About 30% of solar radiation reaching the Earth actually gets reflected straight back out into space, mainly by the white snow & ice of the polar regions, and by the tops of clouds. You can imagine that if the polar ice caps melted, then less energy would be reflected (and vice-versa). But how does the remaining energy (not reflected or absorbed in the upper atmosphere) interact with the Earth’s land surface, oceans and atmosphere? 

It turns out that most of the electromagnetic radiation arriving at the Earth’s surface is absorbed by the surface waters of the oceans, and by rocks, soils and plants on land, making Earth’s surface warmer. The warm surface then re-radiates thermal energy back out towards space as infra-red radiation. 

This is where things gets a bit complicated! It turns out that outgoing infra-red waves interact with molecules in the atmosphere in a very important way. The bonds that hold atoms together in the molecules of “greenhouse gases” like H2O, CO2 and CH4 are flexible and, if ‘pushed’ at the right frequency, start to “wobble” (oscillate) in tune with the infra-red waves, absorbing some of their energy. This is called molecular resonance.

Molecular resonance occurs in response to particular frequencies of radiation, especially in the infra-red. Resonance is familiar to anyone who has heard windows rattle as a heavy lorry drives past, or seen a washing machine start to shudder as it winds down from a fast spin – it occurs when any system is subjected to an external oscillation at its natural vibration frequency.

Molecular resonance (of H2O) is also the means by which microwave ovens cook food! 

Oscillating bonds in a molecule produce vibrating electrical and magnetic fields, and so re-emit their extra energy as more (infra-red) electromagnetic waves. But whilst infra-red radiation from Earth’s surface is essentially heading out into space, the re-radiated energy from our “greenhouse molecules” goes in all directions – some of it back down toward the surface! The more greenhouse gas, the stronger this effect, and so our atmosphere gets warmer, until the energy balance between incoming sunlight and outgoing infra-red is restored.

Don’t worry if you don’t understand the process too well – it’s complicated! The following diagram summarises the effect pretty well! The name “greenhouse effect” comes from its similarity with how the glass in a greenhouse allows light in but prevents heat from escaping.

The greenhouse effect was first described in the nineteenth century, when two scientists, John Tyndall in Britain, and Svante Arrhenius in Sweden, carried out experiments to try and estimate the effect of water vapour and carbon dioxide in raising Earth’s temperature. Tyndall, in the 1850s, realised that, without a natural greenhouse effect, our planet would be ~30 oC colder, permanently frozen and unable to sustain life! Arrhenius, in the 1890s, also suggested that the extra CO2 released by the burning of coal might “in a few hundred years” lead to a “noticeable warming” of Earth’s climate. He could not foresee the huge growth in population, and in the use of oil and gas, that would come in the twentieth century. Nor could either Tyndall or Arrhenius foresee that other greenhouse gases, such as methane (CH4) or nitrous oxide (N2O) might also become a serious cause for concern...

Global (and local) climate does not just depend on the circulation of energy by the atmosphere and hydrosphere (mainly oceans*); it is also driven by Earth’s ice-covered regions (cryosphere) and by the many living organisms that constitute the biosphere. 

Water plays a role in all of these Earth spheres – through evaporation from oceans to form clouds, and condensation to produce the rain and snow that form lakes, rivers & ice caps. Water is also vital to all life on Earth and, therefore, to the many processes of the biosphere which, itself has a huge influence of climate, (even without considering human effects!). 

*(The term hydrosphere is taken here to include oceans, lakes & rivers; the cryosphere is described separately)