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Climate in the Anthropocene

New IPCC report shows that humans are driving global warming and subjecting the planet to more extreme weather events, such as heat waves, droughts, and heavy rainfall

Record high temperatures in Europe this summer caused wildfires in Greece

Nicolas Economou / Nurphoto via Getty Images

In 2007, the Intergovernmental Panel on Climate Change (IPCC) issued its fourth report, stating that “warming of the climate system is unequivocal.” Its fifth report, published in 2013, went a step further and stated that “Human influence on the climate system is clear.” On August 9, 2021, the first part of the panel’s sixth assessment report (AR6), which is known as Working Group 1 (WG1) and summarizes the latest scientific knowledge on climate change, was issued, leaving no doubt about the role of contemporary civilization in this phenomenon: “It is unequivocal that human influence has warmed the atmosphere, ocean, and land.” The document also states that “Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities.” Global warming is caused by a progressive growth of atmospheric GHG levels, the main signature left on the climate during the Anthropocene—the epoch of human beings.

No part of the planet is safe from the consequences of the increasing average temperature of the atmosphere and the changes arising from or associated with this process. “Climate change is already affecting every region on Earth in multiple ways. The changes we experience will increase with additional warming,” said Panmao Zhai of the Chinese Academy of Meteorological Sciences, co-chair of AR6 Working Group I, when the report was released. His group was responsible for producing the first part of the IPCC report, which focuses on updating scientific knowledge on climate change. In the first quarter of 2022, the panel is expected to release the next two parts of AR6. The second document will address the impacts and vulnerabilities caused by climate change and what can be done to adapt to them. The third will describe how we can mitigate climate change.

Coordinated by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP), the IPCC does not carry out or fund scientific research. Nor does it suggest environmental policies. However, it does publish a summary of its reports in accessible language, called the Summary for Policymakers, which presents the key scientific findings to help countries plan their approach. The IPCC brings together specialists from different fields to compile and analyze scientific research and produce periodic reports assessing the current situation and making projections on the future of the climate. Three years in the making, the first part of AR6 was written by a group of 234 authors from 66 countries. Another 567 collaborating authors also contributed to the document, which was initially scheduled for release in April but was postponed due to the COVID-19 pandemic. The text, approved by all 195 countries participating in the panel, cites more than 14,000 scientific studies to support its conclusions.

According to WG1, in the early years of the next decade, the atmosphere’s average temperature is “very likely” to increase to 1.5 degrees Celsius (ºC) above the value recorded between 1850 and 1900. The second half of the nineteenth century is considered representative of the planet’s pre-industrial phase, when the atmosphere had not suffered any significant effects of human activity, prior to the burning of fossil fuels that release greenhouse gases. The new date estimated that this 1.5 °C increase will be reached is about 10 years earlier than previous reports had predicted. According to the definitions adopted by the IPCC, a climatic event or result of a process is considered “very likely” when the probability that it will occur is between 90% and 100%. The five future scenarios projected by the IPCC’s climate models, ranging from the most optimistic to the most pessimistic, converge predict that the 1.5 °C global temperature rise will be hit in the 2030s.

Limiting global warming to an increase of 2 °C, preferably 1.5 °C, is the primary objective of the Paris Agreement, signed by 195 countries at the UN in December 2015. Such an increase in temperature is considered high, but the potential socioeconomic impacts would still be manageable. “The IPCC report makes it clear that it will be very difficult to achieve this goal if we do not drastically and quickly reduce greenhouse gas emissions,” says Paulo Artaxo, a physicist at the University of São Paulo (USP), one of the seven Brazilian researchers who contributed to the WG1, and one of the coordinators of the FAPESP Research Program on Global Climate Change (PFPMCG). The IPCC report was released in Brazil during a webinar hosted by FAPESP on August 9.

The Earth is already very close to having heated up by 1.5 °C since the pre-industrial period. The average temperature across the entire surface of the planet has risen by around 1.1 °C—an unprecedented rise over the past 2,000 years, according to the report. The average increase was greater in continental areas where humans live than over the oceans, at 1.6 °C and 0.9 °C respectively. The polar regions, especially the Arctic, have warmed up more than anywhere else since the pre-industrial period, by around 3 °C.

The document also states that if the worldwide trend is not reversed soon, the glaciers will continue to melt, sea levels will continue to rise, oceans will become more acidic, and extreme weather events such as heat waves, heavy rainfall, and severe droughts will become even more frequent and intense over the course of this century. According to studies cited by the IPCC, these phenomena, which have a high potential to cause death and socioeconomic damage, have become more common and more severe in recent decades, and the trend is set to continue as the planet gets warmer.

Extreme heat waves that occurred once every 100 years before the twentieth century, when human influence on the climate was negligible, have now become 4.8 times more frequent and 1.2 °C warmer, according to the IPCC. If the global temperature rises by 1.5 °C, this type of weather event will be 8.6 times more common and 2 °C warmer than 150 years ago, the panel predicts. Another half-degree rise in the planet’s average temperature to 2 °C above preindustrial levels would cause these heat waves to become 13.9 times more frequent and 2.7 °C warmer. If the Earth’s atmosphere warms by 4 ºC, a somber hypothesis, they would be 39.2 times more frequent and 5.2 times warmer than in the last century. The report also details similar trends for severe rainfall and droughts.

The increasingly common occurrence of extreme climate events—such as the record high temperatures of around 50 °C seen in North America and Europe this summer and the largest flood of the Negro River in the Amazon for nearly 120 years as a result of heavy rainfall in the middle of the year—has resulted in a new area of research: specific studies of extreme events to determine whether they were partly or entirely caused by changes induced by humans, or whether they were simply a natural climate phenomenon. “There are still few studies of this type being carried out in South America, but it is a growing area,” says climatologist José Marengo, head of research and development at the Brazilian Center for Natural Disaster Monitoring (CEMADEN), who worked as reviewer and editor of the latest IPCC report (see article).

Reality check
“This report is a reality check,” said Valérie Masson-Delmotte, a French paleoclimatologist from the University of Paris-Saclay and another co-chair of IPCC Working Group 1, in the statement that accompanied the report. “We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done, and how we can prepare.” The task of determining how human activity has contributed to climate change has progressed thanks to a series of improvements in climate studies over the past seven years.

Better paleoclimatic reconstructions, more reliable and detailed data on the evolution of current temperatures and rainfall patterns, and computer models better able to reproduce the past and project the future—all these advances have allowed the panel to reach more incisive conclusions than previous reports. “There is much less uncertainty in this report about what is a result of human activity and what is natural climate variation,” explains oceanographer Letícia Cotrim, from the State University of Rio de Janeiro (UERJ), one of the Brazilian authors of WG1.

Global warming is caused by the emission of greenhouse gases, such as nitric oxide (N2O), methane (CH4), and above all, carbon dioxide (CO2). The new report emphasizes that since the mid-eighteenth century, the increased concentrations of GHGs in the atmosphere is “unequivocally” caused by human activities, especially those associated with the burning of fossil fuels. The conclusion is based on studies that simulated the evolution of the Earth’s average temperature with and without the emission of GHGs by humankind. In scenarios where GHGs were emitted by natural processes only, the planet did not heat up, suggesting that the climate system is in balance. When human contributions were included in the simulations, the planet’s average temperature reached current levels. The current concentration of CO2, the main GHG, is almost 420 parts per million (ppm), about 50% higher than 170 years ago during the preindustrial period.

Once emitted, GHGs have three possible destinations: the atmosphere, the ocean, or the earth’s surface. If stored in the air, they contribute to global warming. In the seas, they make the water more acidic and threaten the life cycle of many species. The most benign path is absorption by plants via photosynthesis, turning the gas into biomass—a tree trunk, for example. Since the industrial revolution, 41% of GHGs have entered the atmosphere and 59% has been absorbed by the land and oceans. “But the atmosphere will continue to absorb more CO2 than the oceans and land surface, both in proportional and absolute terms, if emissions of this gas do not fall by the end of the century,” explains Marcos Heil Costa, a climatologist at the Federal University of Viçosa (UFV) in Minas Gerais who studies the interactions between the climate and agriculture and is one of the WG1 authors. In a pessimistic scenario projected by the IPCC, in which current levels of carbon dioxide emissions double by the end of this century, the atmosphere would store 54% of the CO2 emitted. In an even worse case in which emissions double by mid-century, the amount of CO2 trapped in the atmosphere would rise to 62%.

Climate change intensifies the water cycle, which causes more intense rains as well as more arid and hot periods, leading to more natural disasters in many regions, such as floods, landslides, and severe droughts. For example, with sea levels continuing to rise throughout the twenty-first century, combined with more intense storms, coastal areas will suffer greater erosion and more frequent and widespread flooding. Extreme events associated with rising sea levels that previously occurred once every 100 years could become annual occurrences towards the end of this century. Between 1901 and 2018, the average global sea level rose by 20 centimeters. But in recent years the pace has accelerated, increasing from 1.3 millimeters (mm) a year between 1901 and 1971 to 3.7 mm a year between 2006 and 2018. Continued global warming will amplify the melting of seasonal summer sea ice in the Arctic Ocean and the permafrost—permanently frozen ground in the Arctic region that contains large amounts of methane, a greenhouse gas that if released will further increase the global temperature.

Each 0.5 ºC increase in the planet’s average temperature has a non-linear effect on the frequency and intensity of heat waves, droughts, and heavy rains. “A temperature increase of this magnitude would double or triple certain extreme weather events,” points out Lincoln Muniz Alves, a climatologist from the Brazilian National Institute for Space Research (INPE) and another member of IPCC Working Group 1. Alves was part of the team that developed an interactive atlas, something new for the IPCC report, which can be accessed online.

Ulrik Pedersen / Nurphoto via Getty Images  Polar glaciers, like those in the Arctic, are melting as a result of global warmingUlrik Pedersen / Nurphoto via Getty Images 

Effects in Brazil
The IPCC report does not make specific projections for individual countries, only for the planet as a whole or particular regions. These regional projections, however, can give us an idea of the current and future climate trends in South America and Brazil according to the scientific literature. The average temperature on the continent is generally rising at a faster rate than the global average. The sea level in the South Atlantic (but not in the Pacific) has risen more sharply than the global average in recent decades, a trend that is expected to continue throughout this century, leading to increased coastal flooding. If the global temperature increases by 2 °C or more, projections for Brazil’s vast territory include both heavier rainfall and more droughts, in a kind of climate seesaw with potentially disastrous impacts.

In the Amazon, estimates suggest with a high degree of certainty that there will be more days without rain and more droughts, and with a lower level of confidence that there will be more episodes of extreme rainfall and floods. In the worst-case scenario, where global CO2 emissions double by 2050, temperatures in the region will reach 35 ºC on more than 150 days a year by the end of the century. For the Northeast, which is naturally much drier, the outlook is similar, with the length of the dry season expected to increase.

In the Midwest, there are no reliable data suggesting a decrease in the amount of rainfall, but the rainy season is likely to start later in the year. More extreme rainfall events and droughts are expected, in addition to more wildfires, which will affect agriculture and biomes such as the Pantanal and the Cerrado. In the South and Southeast, where 60% of the Brazilian population lives, there has been a clear increase in the average rainfall and extreme rainfall since the 1960s, trends that will intensify in the coming decades if the planet warms by 2 °C. “Temperature projections are always more reliable than those for rainfall,” explains Alves. This is because global warming is a more direct process based on fewer variables than cloud and rain formation. “Modelling the formation of clouds in the atmosphere and their interactions with other climate parameters is still a challenge,” says Marengo.

It is no coincidence that there is more and better information on temperature increases than rainfall for practically every region of the world. The IPCC atlas divides the inhabited sectors of the continents into 45 areas, 41 of which experienced more heat waves now than they did in 1950. In most of these areas, there is a medium or high degree of certainty that human activities have contributed to this phenomenon. For episodes of heavy rain, an increase was only observed in 19 regions in the same period, almost always with a low degree of certainty that humans have played a role in the trend. In the remaining 26 areas, data was insufficient or not of high enough quality to establish a trend. “The atlas makes it clear that most climate studies and data are concentrated in countries of the Northern Hemisphere. We need to invest more in research on climate change in South America and Brazil,” concludes Alves.