Impacts of climate change

Different crops demand different conditions to grow. Even within a country like Britain, crops such as apples grow better in some places than in others. This regional variation is often down to different local conditions, including climate. If temperatures rise, some crops may become harder to cultivate and may have to be replaced with ones more suited to the new environment.

In the future, a warmer climate may cause more heatwaves, increase rainfall and increase the frequency and strength of storms. Over the past 35 years, there has been an almost fivefold increase in the recorded number of disasters caused by natural hazards, although not all were influenced by extreme weather events (source: III.ORG[1]).

As the Earth warms up, interactions between the oceans and atmosphere can amplify the frequency and intensity of weather events. These include hurricanes, floods and droughts. Extra water vapour in the atmosphere falls again as extra rain, which can cause flooding. During hot weather, evaporation from both land and sea increases and can cause droughts in regions where there is low precipitation.

Worldwide, glaciers are shrinking rapidly and ice appears to be melting faster than previously estimated. In areas dependent on meltwater from mountains, changes in water flow and availability can lead to a lack of water and even drought.

According to the Intergovernmental Panel on Climate Change (IPCC[2]), up to a sixth of the world’s population lives in areas that will be affected by meltwater reduction. In some regions of the world, this will result in crop failure and famine, especially in areas where temperatures are already high.

Increased levels of ice and snow melt cause sea levels to rise, as does thermal expansion (water expands when warmed). Areas that lie just above sea level now, may one day become submerged. For example, some Pacific Island nations are expected to be partially or completely submerged by the end of the 21st century.

A rise in sea levels will also have an impact on coastal and shallow marine plants and animals will be affected, for example mangroves and coral reefs. In countries with large areas of coastal lowland, there will be a dual risk of river floods and coastal flooding, which will reduce the area for living and working. Coastal defences will need strengthening and river levées will require developing.

An increase in standing water may potentially increase populations of insects like mosquitoes, and therefore malaria, and other diseases that are associated with standing water, such as Lyme’s disease.

Wildlife will respond to climatic changes in different ways. Certain species might be able to adapt or migrate to habitats with more suitable conditions. Where species are unable to adapt and conditions become inhospitable, populations are likely to become threatened and, in extreme cases, species may face extinction.

In 2019, a UN report suggested that around one million animal and plant species would be threatened with extinction, many within decades, and more than ever before in human history.

The characteristics of different rocks depend on the environment in which the sediments that form them were deposited.

• Some sands and gravels are deposited by retreating glaciers and they become a distinctive sediment called ‘till’. Where till is found, we know that there must once have been glaciers and therefore a cold climate.
• Rocks that form in a hot desert environment are often coloured red by iron deposits.
• In regions with high temperatures, water can evaporate quickly leaving behind a layer of salt on the ground that becomes preserved in the rocks and is another indicator of a hot climate.

Different species of plants and animals need different conditions to survive. Some can be very sensitive and do not adapt easily to change. For example, coral reefs live in tropical waters within a particular temperature range, specific depths and require certain light levels. If the water conditions change, even by a relatively small amount, they cannot survive. Therefore, wherever fossil corals are found, it is possible to estimate the environment in which they lived in the past because of these specific environmental requirements.

Pollen and spores

Plants produce pollen and spores that are particularly useful in helping to determine climate.

Pollen and spores are produced in their millions. They have a strong outer case, so they are easily preserved as fossils if they are quickly covered in mud. Each plant has different shaped pollen or spores so it is possible to identify the type of plant it came from under a microscope.

Different plant species require different climatic conditions, therefore looking at the pollen in a layer of rock can be a good indication of the climate at the time when they were living.

Microfossils

Microfossils are tiny fossils, usually smaller than 4 mm in size. They can be the remains of minuscule life forms or microscopic parts of plants and animals. Like other plants and animals, they may require specific living conditions, but they are more abundant and are found in many types of sedimentary rocks. Because of this they can be a useful indicator of how climate has changed over a period of time.

Today’s landscape gives clues to the climate of the past. For example, glaciers leave tell-tale signs of their activity. As glaciers move slowly down river valleys, they can carve out deep, U-shaped valleys such as the Norwegian fjords. Glaciers also pick up pieces of rock and gravel and, as they move forwards, this debris scratches grooves called ‘striations’ into the rocks on the valley floor.

When the ice melts, the very large pieces of rock they have been carrying are dumped, often many miles from where they were picked up. These abandoned boulders are called ‘erratics’.

All of these kinds of erosion and deposition clearly tell of temperatures much colder than our current climate.

There is more than one type of oxygen and these different types are called isotopes. Some of these isotopes are ‘heavier’ than others and this mass is determined by the number of neutrons the isotopes contain. Oxygen-16 (containing eight protons and eight neutrons in each atomic nucleus) is lighter than oxygen-18 (eight protons and ten neutrons). These isotopes can also be written as ¹⁶O and ¹⁸O.

Antarctic ice cores hold the secrets of our past climate. A European project revealed a climate record for the last 800 000 years. It showed that the Earth experienced eight glacial cycles — each with a warm period and an ice age. Source: British Antarctic Survey.

In water, the relative amount of each of type of oxygen isotope varies with the temperature. Rain and ice contain water high in ¹⁶O so when there is lots of ice, the ocean water is left with a lot of ¹⁸O.

Shells and sediments carry the isotopic signature of the water in which they were formed so, by analysing the ratio of ¹⁶O to ¹⁸O recorded in rocks, fossils, ice and sediments, we can find evidence of the climate at the time they were formed.

One of the warmest periods in the Earth’s history was the Cretaceous, from 145 to 66 million years ago. The Earth was then several degrees warmer than today and is described as having a ‘greenhouse’ climate. Britain enjoyed warm, tropical conditions at this time. The poles were warm and at times there may have been no ice on them at all. There is even evidence of temperate forests growing in the Arctic and Antarctic. As ‘greenhouse’ temperatures were reached, the world’s ice melted, which caused significant sea level rise.

During the late Cretaceous, the high levels of atmospheric CO₂ heated up the Earth in the same way as they are doing today. Even high latitudes were affected, with land areas becoming covered by lush, subpolar forests. Ice was largely absent from our planet. At this time, thick deposits of chalk were accumulating in warm seas across southern England. This rock, which now forms the famous white cliffs of the Sussex and Kent coast, contains microscopic fossils that preserve a chemical signal of the sea temperature in the late Cretaceous.

This evidence shows that the seas and oceans were becoming progressively warmer, with sea-surface temperatures around Britain probably reaching 28°C. Even deep ocean water seems to have been warm, perhaps as high as 15°C, compared with the near-freezing deep ocean water of today.

A critical point was reached about 93 million years ago, at a time called the Cenomanian/Turonian boundary by geologists. In all the deep oceans of the world, and some of the shallow seas, there was a sudden appearance of dark mudstones containing high levels of organic carbon.

Normally, when an organism dies, its organic matter decays as other life forms consume its nutrients and carbon. When oxygen is largely absent in this process, substantial amounts of concentrated organic matter becomes preserved. This may occur in isolated lakes and seas that are cut off from fresh supplies of oxygenated water

At the Cenomanian/Turonian boundary, all the deep oceans and some shallow seas appear to have become temporarily oxygen starved. This event is known as an ‘oceanic anoxic event’ and the organic-rich muds that formed are called ‘black shales’. The seas covering southern England were too shallow and aerated to form black shales at this time, but they did form in the deeper water covering Lincolnshire and Yorkshire, where a dark mudstone known as the ‘Black Band’ represents this event.

Oxygen starvation of the oceans on a global scale seems an improbable event, but in 2007, an area of the Pacific Ocean off the north-west coast of the USA experienced just such a fate. In this case, the decomposing remains of an unusually rich bloom of plankton consumed the oxygen in the water. Fish, crabs and marine worms all perished in a brief period of anoxia.

In the late Cretaceous, the oxygen starvation lasted much longer, perhaps between 400 000 and 800 000 years, although occasional pale-coloured beds in black shale horizons suggest brief periods of better oxygenation. The cause of the late Cretaceous oceanic anoxic event is still uncertain, but it seems to be related to a rise in global sea levels. This may have triggered an explosion of life in near-surface waters and/or funneled poorly oxygenated saline currents from newly created shallow seas into deep ocean basins. Certainly the high temperatures of both surface and deep ocean waters suggest a limited holding capacity for dissolved oxygen compared to modern oceans.

There is a balance between the gases in the atmosphere and the gases dissolved in the oceans. CO₂ consumed in the ocean by photosynthesising plankton (phytoplankton) is quickly returned to the ocean when they decay or when they are consumed as part of the food chain. In this way the ocean/atmosphere balance is maintained.

However, in an oxygen-starved ocean, organic matter is not recycled and instead accumulates to form black shales. The ocean becomes depleted in CO₂ and is no longer in balance with the atmosphere. Theoretically, this balance could be restored by more atmospheric CO₂ being drawn down into the oceans. It is thought that oxygen starvation of the oceans in the late Cretaceous allowed the oceans to lower CO₂ in the atmosphere and initiate global climatic cooling.

Whilst the cause of oceanic anoxic events might still be unresolved, there is growing evidence that they represent Earth’s natural response to ending extreme greenhouse climates. Study of the chalk cliffs of southern England shows a progressive warming trend, evidenced in microscopic fossils. The deposition of black shales in the deep oceans also show a long term cooling trend afterwards. This suggests falling atmospheric CO₂ levels.

The locking away of organic matter in black shales prevented it from being recycled to the oceans to sustain new life. As the dissolved CO₂ in the oceans became used up, more was absorbed from the atmosphere. Since fresh volcanic supplies of CO₂ had now dwindled, the effect was long-term cooling down of our planet for the remainder of the Cretaceous.

The continued study of ancient greenhouse climates offers us one of the best means of predicting what the future effects of modern global warming might be.

Sea level, or ‘mean sea level’ as it is sometimes known, is the average height of the ocean’s surface between high and low tide. Changes in tides and wave conditions over time are averaged out to determine a ‘still water level’ that can be used to identify whether the sea level has changed and the height of the land above sea level.

Because the surface of the land also moves up and down over long periods of time, sea level changes may be described as ‘relative’ or ‘absolute’. Relative changes may be observed even though the absolute height of the oceans remains the same.

It is important to realise that, when earth scientists talk about a rise in mean global sea level (what we call ‘eustatic’ sea level), local changes may be quite different. This is why local studies of sea-level rise and fall are so important.

During the last ice age, lots of sea water was stored in ice sheets and glaciers and on average, sea level was more than 130 m lower than it is today. Sea level changes related to the growth and melting of ice sheets happen relatively quickly, taking tens of thousands to hundreds of thousands of years, but sea levels also rise and fall slowly over millions of years. These gradual changes have generated the thick sequences of sedimentary rocks that record the long history of geological time. We would have very little, if any, of our sedimentary rocks were it not for the fact that sea levels rise and fall.

Sea levels change for many reasons and over different timescales. Sea levels are rising today not only because significant parts of the land-based ice is melting, but because ocean waters are warming and expanding.

Note that there is no direct effect on sea level when sea-based ice melts, because sea ice is already part of the ocean system. Ice floats because it is less dense than water and, because it displaces an equivalent mass of water, there is no increase in sea level as it melts — in just the same way a cold drink does not overflow the glass as the ice cubes it contains melt. There is, however, a significant indirect effect of Arctic ice melting.

‘Albedo’ is the ability of a surface to reflect sunlight and ice and snow are very good at reflecting the warming rays of the sun. As the Arctic ice melts, polar regions will warm even more quickly because the Arctic ocean will now absorb the incoming energy. Thus, warming leads to Arctic ice melting, which leads to more warming. This is a classic positive feedback in the climate.

Coasts are sometimes characterised by wetlands; great expanses of swamps and marshes occupied by a particular ecosystem that is suited to the brackish conditions. These environments can be preserved by being buried under sand and beach deposits as sea level either rises or as the coastal region sinks. Later in time, we might find evidence of these former environments (or palaeoenvironments) through rock cores or in the faces of quarries.

Their presence tells us of an older sea level and ,if we can measure their ages (for example, through radiocarbon dating techniques), we can build up a history of sea level changes.

About 50 million years ago, in southern England and the Isle of Wight, mangroves grew in the wet areas and, in the drier areas, the plants included pines, laurel, raspberry and magnolia. Laurel is widespread today, in tropical, subtropical and Mediterranean regions.

Marine terraces and buried wetlands are only two of many types of geological evidence for earlier sea levels. Geologists also study foraminifera and diatoms for evidence of ice ages and sea level change. These fossils can be used as indicators of past climates, as different species prefer living in different water temperatures.

Many marine organisms, including foraminifera, have hard shells made of calcium carbonate. The composition of oxygen isotopes found in the carbonate shell can be measured to determine what the water temperature was like when the organism was formed. High proportions of the ¹⁸O isotope in the shell are associated with cold seas and glaciation, because relatively more of the lighter isotope, ¹⁶O, is removed from ocean waters by the process of evaporation and subsequently locked up in snow and ice during cold periods.

Diatoms are single-celled algae encased in silica and live in almost any freshwater or marine environment.

Foraminifera and diatoms are useful in the reconstruction of past sea levels because, in coastal environments, their presence is controlled by the tides. Both are also found in high numbers and preserve well in the fossil record. If their relationship with the varying tides can be worked out, past sea level can be reconstructed from their abundance in sediment cores. Many of these species reside in salt marshes, where there is ample organic material available for radiometric dating. These dates can be used to cross-check rates of sea-level change in the past.