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 always remained well below 300 ppm; in 2022 it passed 420 ppm and continues to rise.

[More detail: try 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)

Climate drivers:

1. The Atmosphere:

Quite apart from its role in transporting energy and water via weather systems (see above), our atmosphere provides us with vital oxygen to breathe, and also the carbon dioxide necessary for plants to grow. In addition, it blocks harmful radiation like X-rays from reaching Earth’s surface and, through the natural greenhouse effect of gases like carbon dioxide and water vapour, keeps our planet warm enough for life to flourish. Our atmosphere has long been kept in balance by its interaction with the oceans, living organisms and geology (volcanic activity, weathering etc.) that, between them, recycle important elements such as carbon and nitrogen through various chemical and biological processes. 

2.  The Hydrosphere:

There are almost 1.4 billion km3 of water in the world’s oceans, which cover 70% of our planet’s surface, and so absorb the majority of solar energy. Water has an astonishing capacity to store thermal energy (heat) – it takes 4,200 J of energy to raise the temperature of 1 kg of water by just 1 oC. If you want, you can even work out how much energy it would take to heat the oceans by just one degree, but the number (6 x 1024 J) is too big to imagine. The point is, that the oceans have to absorb a huge amount of energy before there is a noticeable rise in temperature, which is one reason why many people have taken so long to accept that global warming and climate change may actually be happening! The (very simplified) diagram of ocean currents in the box above also shows how surface currents transport thermal energy from the tropical regions towards the poles, which explains why the effects of global warming are being felt most strongly in high-latitude (Arctic and Antarctic) regions. As cold, deep-water currents eventually return to the surface, they also bring up nutrients vital to marine life.  

Four other effects of global warming on the oceans are worth mentioning here; all of them with important implications for our future climate and food supplies:

• rising sea level, as a result of melting ice sheets (see below) and thermal expansion of sea water as it warms. Estimates suggest a likely rise of up to 50 cm by 2050, and 2 m by 2100, which would drown many port cities around the world along with huge areas of productive agricultural land, and could also create many millions of climate refugees.

• increasing occurrence & severity of storms: hurricanes are typically generated when sea surface temperatures exceed 26 oC in tropical oceans, so higher sea temperatures, plus increased humidity due to evaporation would be expected to form more storms.

• ocean acidification: many marine organisms like corals make their shells by combining calcium (Ca) with dissolved carbonate (CO32-) ions in the surface waters of the sea. Over the (very) long term, this acts as a “sink” for atmospheric carbon dioxide. However, there is a delicate balance that is easily upset: an excess of atmospheric CO2 dissolving in sea water forms bicarbonate ions (HCO3-) which make the sea water more acidic. This makes it harder for organisms to build new shell material, and can even start to dissolve away existing carbonate shells – already happening in some places.

• deoxygenation: we are used to the idea that things dissolve better in warm water than in cold water, but this is not the case for dissolved gases such as carbon dioxide or oxygen. The decreased solubility of oxygen in warmer oceans has profound implications for marine life, and for future fisheries, and rising sea temperature also reduces the capacity of the oceans to remove atmospheric CO2.

3.  The Cryosphere

This term refers to the frozen regions of the world – ice caps, glaciers and tundra regions where the ground is permanently frozen (permafrost). Just as water has a high heat capacity (see above), so ice has a large Specific Latent Heat value, meaning that it takes a lot of energy to melt ice, another delaying factor in global warming and climate change. 

The recent warming of our planet is having three main effects:

• Reduced albedo (proportion of sunlight reflected back into space), as sea ice in the polar regions is melting and glaciers are retreating, allowing more of the Sun’s energy to be absorbed and increasing global warming (see “Climate Feedback” below).

• Rising sea level caused partly by melting of ice caps (Greenland & Antarctica) and glaciers on land, as well as thermal expansion of the oceans (see above). Note that melting sea ice does not (directly) cause sea level rise – can you work out why?

4. The Biosphere:

We depend upon our living world - the Biosphere – for all of our food and many of the other things we use in our lives. The biosphere also influences global climate through its interactions with the oceans and atmosphere, for example, by the way in which plants & algae remove CO2 from the atmosphere and oceans by photosynthesis, whilst volcanoes, wildfires, & decaying vegetation replenish it as part of the carbon cycle. Other natural cycles in which the biosphere plays a vital role include the water cycle, nitrogen, sulphur, oxygen, phosphorus and more.

Human activity is disturbing the biosphere in many ways, and at an ever-faster rate. We are familiar with the idea that vanishing rain forest in places like the Amazon basin means that less atmospheric CO2 is removed by photosynthesis, and also threatens the existence of many species of plants and animals, but there are other knock-on effects, too. In rainforests, for example, trees play a vital role in recycling water via transpiration, so that rain in one part of the forest leads to increased humidity and more rain in another part. Losing significant tracts of forest risks drying out land areas far beyond the forest itself. 

Once land has been cleared for farming, the most profitable way to use it is often through production of single crops, known as monoculture, or for large-scale cattle ranching. 

Monoculture offers economies of scale and efficiency, but is damaging to the environment because it tends to devastate local biodiversity and often requires huge inputs of pesticides & fertilizers. Clearing and cultivating soils, or applying nitrogen-rich fertilisers, causes emission of nitrogen compounds including nitrous oxide(N2O), which is another very powerful greenhouse gas (about 300x as effective as CO2!). Soils, of course, have been releasing these nitrogen compounds into the atmosphere for hundreds of millions of years, to be gradually recycled via the nitrogen cycle; what has changed (by several orders of magnitude) is the rate at which nitrogen oxides are being released – a rate far greater than nature can manage. 

The biosphere is, of course, highly reliant on water – indeed, most of it lies with the world’s seas and oceans. On land, we have a good idea of how human activity is affecting the availability of fresh (but increasingly polluted) water for agriculture or for our own consumption in some cases, we can see how over-use of land and water resources leads to desertification – when once-productive agricultural land is turned into a “dustbowl”.

Climate Feedback

The discussion of factors affecting Earth’s climate has already suggested that some changes tend to be self-sustaining. A simple example is the albedo effect mentioned in the Cryosphere section above.

The idea here is that if global warming is sufficient to cause significant melting of sea ice (e.g. Arctic ocean) and glaciers on land, then less sunlight is reflected back into space by the snowy white ice surface, and so more of the Sun's energy is absorbed at the Earth's surface, and global warming increases. This is called a positive feedback loop. 

Note: in a cooling scenario (at the start of an ice age), this same positive loop works in the opposite way as more ice forms, increasing albedo and leading to further cooling. 

One of the problems of global warming is that several positive feedback loops may operate at the same time. We have seen that, as the arctic warms, methane (CH4) is released from melting permafrost, whilst increasing sea temperatures may also release CO2 into the atmosphere as its solubility in water decreases. Both of these greenhouse gases then act to amplify the warming effect.

A simple example of negative climate feedback can also result from global warming, since a warmer atmosphere and oceans must lead to increased evaporation and cloud formation. Although the role of clouds is more complex than suggested here, it is easy to see how their white tops would reflect more energy back to space and therefore help to cool Earth’s climate or, at least, to reduce the warming that led to increased cloud.

The worry here is that we might reach a Tipping Point where feedback might set off further warming events one after another.

If you ever wondered why Climate Scientists are obsessed by 1.5 oC, now you know!

Lessons from the past:

We know that our planet must have maintained a climate suitable for the evolution of life for thousands of millions of years – after all, that is how we come to be here! This does not mean, however, that our planet has remained unchanged; in fact, every burst of evolution has tended to follow on after some sort of catastrophe – usually involving climate change! 

The event that most people know about is the impact of a large meteor that caused the rapid extinction many species, including dinosaurs, about 65 million years ago. An initial cooling effect, caused by material flung high into the atmosphere and blocking out the Sun, was probably followed a few months later by warming due to the carbon dioxide and methane that was released as a result of the impact and ensuing forest fires, and also from volcanic activity.

 A much greater extinction event – often called the “Great Dying” by Earth scientists, occurred around 250 million years ago, in which over 90% of all species died out. In this case, the culprit seems to have been a huge series of volcanic eruptions in what is now Siberia, that covered vast areas in lava and, more importantly, released enough CO2 into the atmosphere to set off a runaway greenhouse effect and cause serious ocean acidification and deoxygenation. Global temperature may have risen by 8 oC or more. Marine life was all but wiped out, whilst life on land fared little better. This extinction event probably took about 80,000 years to unfold – to an Earth scientist, this is “in the blink of an eye”, but climate is now changing many times faster!

The events just described are two of five major extinction events (below) to affect planet Earth over the last 550 million years (since fossils became common). Are we on the brink of a sixth event? That’s up to us!

Tackling Climate Change:

If you have reached this point in the Guide, then congratulations! More importantly, you will have understood why more and more people are now taking climate change very seriously and looking for ways to tackle it. In recent years, the term "Net Zero" has been adopted, by politicians and the press, to express the need to cease adding greenhouse gases to our atmosphere - both by decreasing our emissions, and also by helping Nature to "lock down" (sequester) carbon in soils or underground. 

We each have an important role to play: perhaps the most important thing is to question the effects of our everyday actions on the environment and try to act in a way that does least damage. This could mean eating less meat, spending less time in the bath or shower, trying to buy more “environmentally friendly” products, recycling all you can, and so on. When the TV presenter, Sir David Attenborough, was asked what was the most important contribution that everyone could make to “saving the planet”, he answered very simply: “Waste less”. 

In future, we will need a lot more scientists and engineers to work on further ways to “save the planet”, and more qualified technicians to build, install and maintain the new technologies. In future, many industries must respond to those demands, with new products and new ways of working. 

Much more detail about Net Zero is contained in the presentation, "Tackling Climate Change: To Net Zero and Beyond" on this website, and in Part 2 of this Climate Guide.

© MDP 2023