Fifth IPCC shows intensification of climate change

Document affirms that the planet's temperature could rise by almost 5 °C during this century, which could raise sea levels by 82 centimeters.

By Karina Toledo

Agência FAPESP – If greenhouse gas emissions continue to grow at the current rate over the next few years, the planet’s temperature could increase by 4.8 degrees Celsius during this century, which would result in an 82-centimeter increase in sea levels and cause significant damage to several coastal regions around the globe.

This warning came from the scientists of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), which released the first part of its fifth assessment report (AR5) in Stockholm, Sweden. Based on the revision of thousands of studies conducted over the last five years, the document presents the scientific bases for global climate change.

According to Paulo Artaxo, a professor at the Universidade de São Paulo’s Physics Institute (IF –USP) and one of six Brazilians who participated in the preparation of this report, four different possible scenarios for the concentration of greenhouse gases through 2100 were simulated. These four scenarios are the so-called “Representative Concentration Pathways (RCPs).”

“Two basic ingredients are necessary to predict the increase in temperature: a climatic model and an emissions scenario. In the fourth report (released in 2007), researchers also performed four simulations but took into consideration solely the quantity of greenhouse gases emitted. In this fifth report, we used a more complete system that takes into consideration the impacts of these emissions, i.e., how much of an alteration will occur in the balance of radiation in the terrestrial system,” explained Artaxo, who was in London for FAPESP Week London, where he participated in a panel discussion on climate change.

This radiation corresponds to the proportion of solar energy that reaches and leaves our planet, indicating what is stored in the terrestrial system, in accordance with the concentration of greenhouse gases, aerosol particles emitted and other climate agents.

The most optimistic scenario forecasts that the Earth’s climate system will store an additional 2.6 watts per square meter (W/m2) of radiation. In this case, the increase in the Earth’s temperature could be between 0.3 °C and 1.7 °C from 2010 to 2100, and the sea level could rise by between 25 and 55 centimeters by the end of this century.

“In order for this scenario to occur, greenhouse gas emissions would have to stabilize in the next 10 years and action would have to be taken for their removal from the atmosphere. Even then, the model indicates an additional increase of almost 2 °C in the temperature – beyond the 0.9 °C that our planet has already warmed since 1750,” evaluates Artaxo.

The second scenario (RCP4.5) forecasts storage of 4.5 W/m2 of radiation. In this case, the increase in temperature would be between 1.1 °C and 2.6 °C, and the sea level would rise by between 32 and 63 centimeters. In the third scenario, which calls for the storage of 6.0 W/m2 of radiation, the temperature increase would be between 1.4 °C and 3.1 °C, and the sea level would rise by between 33 and 63 centimeters.

In the worst case scenario, in which emissions continue to grow at an accelerating pace, the storage of an additional 8.5 W/m2 of radiation is forecast. In that situation, according to IPCC, the temperature of the surface of the Earth will increase by between 2.6 °C and 4.8 °C during this century, causing the sea level to rise by between 45 and 82 centimeters.

“The sea level has risen by 20 centimeters between 1900 and 2012. If it rises another 60 centimeters, coupled with the tides, the result will be major erosion of coastal areas worldwide. Rivers like the Amazon, for example, will experience a strong reflux of salt water, which will affect the entire local ecosystem,” said Artaxo.

According to the IPCC’s AR5 report, in all of the scenarios, it is very probable (90% probability) that the rates at which sea levels rise over the 21st century will exceed the rates at which they rose between 1971 and 2010. Thermal expansion resulting from the increase in temperatures and the melting of the ice caps would be the main causes.

The warming of the oceans, says the report, will continue to occur for centuries, even if greenhouse gas emissions diminish or remain constant. The Arctic region is predicted to warm the most, according to the IPCC.

According to Artaxo, the warming of the oceans will have additional significant consequences that have not been considered in previous climate models. As the oceans warm, they lose the capacity to absorb carbon dioxide (CO2) from the atmosphere. If emission levels are maintained, however, there will be an increase in the concentrations of this gas in the atmosphere.

“In the previous report, the chapters dedicated to the role of oceans on the climate lacked experimental data. But in the last few years, there was an enormous advance in climate science. In this fifth report, our confidence in the impact of oceans greatly improved because of the measurements conducted by satellites and observations conducted with networks of buoys – like those in Projeto Pirata, which FAPESP funds in the South Atlantic,” affirmed Artaxo.

Acidification of oceans

In all scenarios forecast in the fifth IPCC report, the concentrations of CO2 in 2100 will be greater than the current levels, as a result of the cumulative increase in emissions during the 20th and 21st centuries. Part of the CO2 emitted by human activity will continue to be absorbed by oceans, and therefore, it is “virtually certain” (99% probability) that acidification of oceans will increase. In the best case scenario – RCP 2.6 – the decrease in pH will be between 0.06 and 0.07; in the worst case scenario – RCP 8.5 – it will be between 0.30 and 0.32.

“Sea water is alkaline, with a pH of approximately 8.12. But when it absorbs CO2, there is a formation of acid compounds. These acids dissolve the carcass of part of the marine microorganisms, which are generally made of calcium carbonate. The majority of the marine biota will face profound alterations that affect the entire food chain,” affirms Artaxo.

By analyzing the changes to date, the IPCC scientists affirmed that the last three decades were warmer than all of the previous decades since 1850. The first decade of the 21st century was the hottest of all. The period between 1983 and 2012 was “very probably” (90% probability) the hottest in the last 800 years. There is also a 60% probability that this period was the hottest of the last 1400 years.

Still, the IPCC recognizes that there has been a decrease in the global warming rate in the last 15 years, from 0.12 °C per decade (considering the period from 1951 and 2012) to 0.05 °C (when considering only the 1998 and 2012 period).

According to Artaxo, this decrease is due to two main factors: the greater absorption of heat in deep water (more than 700 meters) and the greater frequency of La Niña, both of which alter the rate of heat transfer in the ocean atmosphere. “The process is very clear and documented in renowned scientific magazines. Still, the planet continues to warm significantly,” she said.

There is a 90% certainty that the number of cold days and nights has diminished, while the number of hot days and nights has increased on a global scale. There is also approximately a 60% certainty that heat waves have increased. The report says that there is evidence of ice melting, mainly in the Arctic region. There is a 90% certainty that the rate of reduction in the size of the polar ice caps has been between 3.5% and 4.1% per decade between 1979 and 2012.

The concentrations of CO2 in the atmosphere have increased more than 20% since 1958, when systematic measures began, and approximately 40% since 1750. According to IPCC, these increases are the result of human activity, mainly the burning of fossil fuel and deforestation, with a small participation from the cement industry.

For the scientists, there is very high level of confidence (90%) that the average CO2, methane and nitrous oxide concentrations are the highest of the last 22,000 years. However, changes in solar irradiation and volcanic activity have contributed a small fraction to climatic alteration. It is “extremely probable” (95% certain) that human influences on climate have caused more than half of the increase in temperature observed between 1951 and 2010.

“The effects of climate change are already being felt; they are not only something to expect in the future. The increase in the frequency of heat waves, hurricanes, floods and severe storms and brusque variations between hot and cold days are probably related to the fact that the climate system is being changed,” said Artaxo.

According to the IPCC evaluation, many aspects of climate change will persist for many centuries if greenhouse emissions cease. “It is very likely (90% certain) that more than 20% of the CO2 emitted will remain in the atmosphere for more than 1,000 years if the emissions cease,” affirmed the report.

“What we are altering is not the climate of the next decade or that through the end of this century. There are several publications with simulations that show high concentrations of CO2 through the year 3000 because atmospheric CO2 removal processes are very slow,” explained Artaxo.