Climate Change: A Very Short Introduction

by Mark Maslin

the primary source of CO2 is the burning of fossil fuels: over 85% of the global CO2 emissions comes from energy production, industrial processes, and transport.

The second major source, accounting for 10‒15% of global CO2 emissions, is land-use changes.

Here the pattern of CO2 emissions is different, with South America, Asia, and Africa being responsible for over 90% of present-day land-use change emissions.

We have put nearly half a trillion tonnes of carbon into the atmosphere since the Industrial Revolution, but this still amounts to only half of our total emissions. The other half has been absorbed by the Earth—with 25% going into the oceans and 25% going into the land biosphere. Scientists are concerned that this uptake of our pollution is unlikely to continue at the same level in the future.

This is because as global temperatures rise the oceans will warm and will be able to hold less dissolved CO2. As we continue to deforest and convert land for farming and urbanization, there will be less vegetation to absorb CO2, again reducing the uptake of our carbon pollution (Figure 6).

Since 1880, the global average surface temperature has increased by 1.1°C.

Since 1850, the land has warmed by 1.44°C and the oceans by 0.89°C

The IPCC has also compiled all the current data on global sea level. They show that between 1901 and 2018, the global sea level rose by over 24 cm

The best satellite data started in January 1993 and show a trend of over 35 mm rise in sea level per decade.

The sea-level rise of the last 30 years is made up of the following contributions: 39% from thermal expansion of the ocean; 9% Antarctic ice sheet; ~12% Greenland ice sheet; 27% glaciers and other ice caps; and another ~13% due to the overall reduced land storage of water

At the moment it is estimated that Greenland is losing over 230 gigatonnes (Gt) of ice per year, a seven-fold increase since the early 1990s.

The ice cover records from the Tornio River in Finland, which have been compiled since 1693, show that the spring thaw of the frozen river now occurs a month earlier.

The latest IPCC report states that it is virtually certain that anthropogenic climate change has caused increases in the frequency and severity of hot extremes and decreases in cold extremes on most continents.

The past decade has seen record-breaking heatwaves occurring in Australia, Canada, Chile, China, India, Japan, the Middle East, Pakistan, and the USA.

Record-breaking extreme floods have been recorded over the past decade in Brazil, Britain, Canada, Chile, China, East Africa, Europe, India, Indonesia, Japan, Korea, the Middle East, Nigeria, Pakistan, South Africa, Thailand, the USA, and Vietnam.

For example, in 2019 there were four spectacular cyclones in the Indian Ocean and two of these, in the southern Indian Ocean, were unprecedented. The 2018‒19 south-west Indian Ocean cyclone season was the costliest and most active season ever recorded since reliable records began in 1967.

Over 113 extreme weather events that occurred between 2015 and 2020 have been studied using attribution science: 70% of events were found to have increased frequency or intensity due to climate change; 26% were found to have a reduced occurrence due to climate change; and 4% showed no variation due to climate change.

between 1880 and 2020, the 10 warmest years on record have all occurred within the past 15 years, with 2020 joint warmest year with 2016, followed by 2019, 2015, 2017, 2018, 2014, 2010, 2013, and 2005.

The biggest unknown in the models is not the physics or the chemistry or the biology: it is the estimation of future global GHG emissions over the next 80 years.

In fact, the latest (sixth) IPCC Assessment Report (AR6) has compiled the results of multiple runs from over a hundred distinct climate models being produced across forty-nine different international modelling groups, which are all part of the latest (sixth) Coupled Model Intercomparison Projects (CMIP6). Of course, as computer processing power continues to increase, both the representation of coupled climate systems and the spatial scale will continue to improve.

17. The carbon cycle, in gigatonnes of carbon.

What has become clear is that the ocean surface and the land biosphere each take up about 25% of our carbon emissions every year. As the oceans continue to warm they can hold less dissolved CO2, which means their uptake will reduce.

In the most recent IPCC AR6, the past- and future-emission scenarios were used in about 100 distinct, independent GCMs. Each of these models has its own independent design and parameterizations of key processes. The independence of each model is important, as confidence may be derived from multiple runs on different models providing similar future climate predictions. In addition, the differences between the models can help us to learn about their individual limitations and advantages. Within the IPCC, due to political expediency, each model and its output is assumed to be equally valid. This is despite the fact that some are known to perform better than others when tested against the reality provided by the historical and palaeoclimate records. Moreover, though we understand uncertainty within a single model, the notion of quantifying uncertainty from many models currently lacks any real theoretical background or basis.

These climate models suggest that depending on our GHG emissions the global surface temperature could, by 2081–2100, rise between 1.3°C and 5.5°C compared with the pre-industrial period (1850‒1900): see Table 3. In all the scenarios except SSP1–1.9, a global temperature rise of over 1.5°C is reached between 2021 and 2041 with the best estimate being 2030.

As has been the case since the very first IPCC report in 1990, one of the uncertainties in the models is the role of the clouds and their interaction with radiation. Clouds can both absorb and reflect radiation, thereby cooling the surface, and absorb and emit long-wave radiation, warming the surface.

However, it must be remembered that global temperatures will not stop changing once we get to the year 2100.

In February 2005, the British government convened an international science meeting at Exeter, UK, to discuss this very topic. This was a very political science meeting, as the UK government was looking for a recommendation to take to the Group of Eight (G8) meeting in Gleneagles.

The meeting and a lot of supporting research at the time suggested a limit of 2°C above pre-industrial average temperature: below this threshold there seemed to be both winners and losers due to regional climate change, but above this temperature everyone seemed to lose.

At the Paris climate change negotiation meeting in 2015 the Alliance of Small Island States (AOSIS) and some key developing countries reiterated that even a small amount of warming would be devastating for their countries.

Subsequently the IPCC special report on 1.5˚C global warming published in 2018 supported this lower target by demonstrating that there is a significant increase in regional and national climate change impacts between a 1.5˚C and 2.0˚C world.

Heatwaves are often referred to as the ‘silent killer’. They disproportionately affect the elderly, and it is sustained night-time temperatures that kill because while asleep older people are less able to regulate their body temperature.

In 2019, almost three times the global land surface area was affected by extended drought compared with the period between 1986 and 2005.

Climate models indicate an increase in the strength of the summer monsoons as a result of global warming over the next 100 years. There are three reasons why this should occur: (1) global warming will cause temperatures on continents to rise even higher than those of the ocean in summer and this is the primary driving force of the monsoon system; (2) decreased snow cover in Tibet, which is to be expected in a warmer world, will increase this temperature difference between land and sea, increasing the strength of the Asian summer monsoon; (3) a warmer climate means the air can hold more water vapour, so the monsoon winds will be able to carry more moisture.

For the Asian summer monsoon, this could mean an increase of 10–20% in average rainfall, with an interannual variability of 25–100% and a dramatic increase in the number of days with heavy rain.

When hurricanes hit in developed countries, the major effect is usually economic loss, while in developing countries it is loss of life. For example, Hurricane Katrina, which hit New Orleans in 2005, caused over 1,800 deaths and over $150 billion in damages. By contrast, Hurricane Mitch, which hit Central America in 1998, killed at least 11,000 people, made 1.5 million people homeless, and caused $6 billion in damages. And in 2013, Typhoon Haiyan, the most powerful tropical cyclone ever recorded, devastated large portions of South-East Asia, particularly the Philippines, affecting 11 million people, causing over 6,300

the case of Bangladesh, over three-quarters of the country is within the deltaic region formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers. Over half the country lies less than 5 m above sea level, so flooding is a common occurrence.

Examples of current cities most at risk include, in Asia: Dhaka (20.3 million people today), Shanghai (17.5 million), Guangzhou (13 million), Shenzen (12.5 million), Jakarta (10.8 million), Bangkok (10.5 million), Hong Kong (8.4 million), Ho Chi Minh City (8.3 million), and Osaka (5.2 million); in North America: New York (18.8 million), Boston (4.9 million), Miami (2.7 million), and New Orleans (0.4 million); in South America: Guayaquil (2.9 million) and Rio de Janeiro (1.8 million); in Africa: Abidjan (3.7 million) and Alexandria (3.0 million); and in Europe: London (8.9 million) and The Hague (2.5 million).

The main question is whether the world can feed itself with an extra 2 billion people on the planet by 2050 and a rapidly changing climate.

Agriculture production will however reduce significantly in the tropics and sub-tropics due to much hotter temperatures and more variable rainfall.

27. Changes in cereal grain yields between 1980 and 2020.

Here, the total area suitable for growing Robusta coffee would be dramatically reduced, to 10% of the present area, by a temperature increase of 2°C.

Direct measurements of the ocean’s chemistry have shown that the pH of the oceans is getting lower; that is, they are getting more acidic (see Figure 28

This process is controlled by two main factors: the amount of CO2 in the atmosphere and the temperature of the ocean.

With increasing atmospheric CO2 in the future, the amount of dissolved CO2 in the ocean will continue to increase.

The Great Barrier Reef is the world’s largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over 1,400 miles. In other regions, as much as 70% of the coral has died in a single season. There has also been an upsurge in the variety, incidence, and virulence of coral disease in recent years, with major die-offs in Florida and much of the Caribbean region. In addition, increasing atmospheric CO2 concentrations could decrease the calcification rates of the reef-building corals, resulting in weaker skeletons, reduced growth rates, and increased vulnerability to erosion. Model results suggest these effects would be most severe at the current margins of coral reef distribution.

Currently, approximately 2 billion people, one-quarter of the world’s population, live in countries that are water-stressed. It has been suggested that if nothing is done to mitigate climate change then up to 50% of the world population could live in countries experiencing water-stress by 2050.

According to the UN World Food Programme, we currently produce enough food to feed 10 billion people, easily enough to cover the predicted increase in population this century. But there are 821 million people on the brink of starvation today, up by 25 million in just five years.

For example, there is a strong correlation between increased SST and sea level, and the severity of the cholera epidemics in Bangladesh.

Assessments of the potential impact of global climate change on the incidence of malaria suggest a widespread increase of risk because of the expansion of the areas suitable for malaria transmission.

Already in the past five years, the area suitable for malaria transmission in highland areas was 39% higher in Africa and 150% higher in east Asia compared to the 1950s.