What are the impacts of climate change?

Human-caused emissions directly affect plants, as higher CO₂ levels generally increase photosynthesis and plant growth. It is almost certainly because of this ‘fertilising’ effect that land ecosystems take up more than a quarter of the CO₂ emitted by human activities^[1].

Some crops, especially in temperate regions, are expected to grow faster and have higher yields as a result of such increases in CO₂. However, because raised CO₂ levels are also the cause of climate change, their impacts on plants are not straightforwardly positive. While crops in temperate regions could benefit from warmer weather during their growing season as a result of global warming, the effect of climate change impacts such as droughts and heat-stress are expected to have net negative impacts on crops in many warmer regions of the world^[2].

Photosynthesis depends directly on the amount of light absorbed by leaves, and higher CO₂ levels help plants use the light they absorb more efficiently to convert CO₂ into biomass^[3]. In addition, it also makes plants use water more efficiently. This improved efficiency increases vegetation cover – which further increases the amount of light plants absorb. These effects are part of the reason for an increase in green vegetation cover that can be seen from space; although intensive human use of land for growing crops, particularly in China and India, is also contributing to this ‘global greening’, and could account for up to a third or more of observed net increase in global vegetation cover^[4].

Scientists have explored the effects of CO₂ through experiments on ecosystems, including both forests and food crops. They show that increasing CO₂ by a further 150–200 parts per million – up from today’s level of around 410 parts per million – increases the rate of photosynthesis in leaves growing under natural light conditions by around 12% on average^[5],[6].

However, whether specific plants are able to grow faster as CO₂ levels rise also depend on several other factors. Some plants – so-called C₄ plants, including many tropical grasses, maize and sugarcane – have a mechanism that concentrates CO₂ inside their leaves. This means higher CO₂ concentrations will not increase their rates of photosynthesis and their growth. This is why yields for maize are not generally expected to increase^[7] – except in dry areas where crops are not artificially supplied with water, and where the increased efficiency in using water (due to raised CO₂) will benefit growth. Plants’ ability to grow faster in response to increased CO₂ concentrations also depends on whether they can access the extra nutrients they need to grow more^[8].

Non-C₄ crops, like wheat, soybean and rice, can grow more in rising CO₂ concentrations. In one sense, this phenomenon makes increasing CO₂ levels ‘good’ for agriculture. On the other hand, risingCO₂ levels are causing climate change, which is likely to have a harmful effect on crop growth, for example through heat-stress, especially in regions that are already warm. Future scenarios for crop yields suggest that we will see a mixed outcome, with higher average agricultural production in some regions, but increased risks of crop damage in other regions, particularly in the developing world^[9].

The effects of raised CO₂ concentrations on natural ecosystems are also not straightforwardly ‘good’ or ‘bad’. Rising CO₂ levels tend to increase tree cover in grasslands, for example, which is ‘good’ for removing carbon from the air but ‘bad’ for grazing animals, as savannas become less grassy. And as raised CO₂ levels makes plant water use more efficient, plant coverage can increase so much that it ends up using as much or even more water than before – which can add to the pressures on fresh water supplies in ecosystems with limited water^[10].

That a higher level of CO₂ has some positive effects on plant life does not change the fact that continued climate change will have increasingly harmful effects on many aspects of human activity across the globe, including crop growth and agriculture in warmer regions. The outcomes of policies to limit these impacts will take time to come into effect, which makes action towards net-zero emissions an urgent priority.

As the world warms, ice sheets and glaciers on land melt and flow into the ocean. The ocean itself also warms and expands, as it absorbs significant amounts of the heat trapped by the greenhouse gas effect. These changes cause the sea level to rise.

Sea level rise continues to speed up as human-induced global warming increases. Sea levels were rising at a rate of around 8cm per 100 years in the late nineteenth century, 21cm per 100 years in the mid-twentieth century, and now up to around 32cm per 100 years. Future sea level rise depends on how quickly we reduce global greenhouse gas emissions.

However, the time lag between temperature rises and the melting of ice means we are already ‘locked in’ to a certain amount of future sea level rise. In a scenario where emissions are reduced rapidly and the rise in global temperatures stay below 2°C, sea level rise will still reach 29–59 cm in the next hundred years with respect to 1986-2005 levels^[1]. This is because the effect of CO₂ already in the atmosphere has a time lag; it heats the atmosphere slowly.

If emissions continue as they are, and ice-sheets respond to this in an expected manner, sea levels could rise by up by 1 m by 2100 compared to 1986-2005 levels^[2]. This would bring serious risks for coastal regions around the world, including low-lying islands and major cities like Shanghai, Alexandria and Miami. More than half of the world’s largest cities lie along the coast^[3], and just over 1 billion people live in coastal areas within 10 metres of sea level^[4]. Adaptation measures can help protect these areas against serious risks of flooding if they go beyond maintaining today’s standards of protection and prepare for rising sea levels^[5].   

The largest threat of future sea level rise comes from the possibility that the massive ice sheets in the Antarctic and Greenland could melt. In particular, the West Antarctic ice sheet is thought to be vulnerable to collapse. It rests on a bed more than 2 km below sea level and contains enough ice to raise global sea levels by around 3.5 m. In total, there is enough ice on the planet to raise sea levels by 70 m. It is difficult to predict at what level of warming this kind of dangerous change could occur, however the risk grows as global temperatures increase.  

Climate change impacts our society by disrupting the natural, economic and social systems we depend on. This disruption will affect food supplies, industry supply chains and financial markets, damage infrastructure and cities, and harm human health and global development.

The impacts of climate change are already here. Global sea levels have risen 19cm since the beginning of the twentieth century, increasing the risk of flooding for many coastal cities and communities^[1]. Heatwaves and droughts are becoming more common and more intense in many parts of the world, causing harm to human health and more heat-related deaths. Climate change is also affecting food security as rain and heat patterns change. In Southern Europe and some parts of Africa, Asia and South America, crop yields are declining^[2].

In the UK, climate change is making some extreme weather events more frequent and more serious. The winter floods in 2013-14, which cost the economy £450 million in insured losses, occurred due to record rainfall in England and Wales^[3] and were made more likely by climate change^[4]. The European summer heatwave in 2018, which led to wildfires in parts of the UK, was made around 30 times more likely by climate change. Scientists now expect 12% of UK summers to experience the same levels of heat. Before global warming, the risk was less than 0.5%^[5].

As the planet gets warmer, the impacts of climate change will grow. If emissions are not decreased and global warming reaches 4°C by 2100, sea levels in the UK could increase by around 1 m^[6], which would put 3.3 million people at risk of flooding by 2050^[7]. Global food supply would also be less secure as extreme weather events and habitat degradation disrupt supply chains. This could lead to higher food prices and up to 183 million more people in the world facing hunger^[8].

Every bit of warming matters, and vulnerable populations and communities across the world have the greatest difficulty coping with the impacts. Even just half a degree of warming can make the difference between dangerous and manageable effects. By limiting global warming to 1.5°C instead of 2°C, for example, 420 million fewer people would be frequently exposed to extreme heatwaves, and 10 million fewer people would be at risk of flooding from rising sea levels^[9].

These risks and impacts are not evenly distributed, and some regions of the planet will feel the effects of climate change more severely than others depending on their location and ability to adapt. However, because both the climate system and our human societies are globally interconnected, the effects of climate change will impact all countries, companies and communities in some way.

The physical impacts of climate change will affect most aspects of human welfare and the economy^[1].  In some regions of the world, rising temperatures will harm worker productivity and crop yields, and lead to more cardiovascular and respiratory problems and higher mortality rates; although cold-related mortality rates are likely to fall^[2].  Meanwhile, higher sea levels will flood cities, storms will become more damaging, rivers will dry up, glacier-fed water supplies will diminish, and ecosystems will suffer.

Calculations of the costs of these changes are mostly given as a top-level, global average. They are usually given in relation to GDP (‘gross domestic product’) a measure of economic income that is not entirely clear to most of us. GDP is the total economic value of all goods and services produced within a country in a given period; it is used as a standard measure of economic performance.

Some attempts to calculate the total costs of climate change suggest that if the global average temperature increase is kept below 2°C, it will cause economic impacts that are less than 1% of global GDP^[3]. In 2017, 1% of global GDP was $800 billion (£648 billion). If temperatures rise further, however, the costs of climate change would increase rapidly. One study suggests unmitigated global warming could reduce average global incomes roughly 23% by 2100, and make 77% of countries poorer in per capita terms than they would be without climate change^[4].

Calculating economic impacts in these average values, however, hides many of the most important issues related to the costs of climate change. Firstly, calculations are hugely uncertain and often do not represent all major risks. Secondly, economic impacts measured in terms of the effects on global GDP look very different at the level of individual countries, and of individual people.

This is because the social and economic impacts of climate change will be distributed unevenly around the world. Some areas will be completely devastated by climate change, such as low-lying islands and coastal towns, and economies that depend a lot on agriculture. These areas are often the poorest parts of the world, where millions are vulnerable to the impacts of climate change, and their economies will be hit the hardest. People living in poverty are the least able to adapt to warming, and small changes in their income due to climate change-related events can result in overwhelming losses to welfare and livelihoods^[5].  Calculations at the level of GDP can hide these impacts. 

Ultimately, some of the greatest risks are also the ones for which we cannot calculate the costs yet. The climate system has several thresholds and ‘tipping points’, where small differences in temperature can create sudden, unexpected and permanent changes with global consequences. Examples include the collapse of the West Antarctic ice sheet, dieback of the Amazon, failures of the Indian monsoon, and the weakening of Atlantic thermohaline circulation^[6].  

Even though these events are individually unlikely, they would have wide-ranging and serious impacts that represent huge economic and social risks. Major changes like these would increase the potential for conflict, political upheaval and mass displacement^[7].