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09 August 2021 In 2019, atmospheric concentrations of co2 were the highest in the last 2 million years and those of the main greenhouse gases (methane and nitrogen dioxide) the highest in the last 800,000 years; over the past 50 years, the Earth's temperature has grown at a rate that is unmatched in the past 2,000 years; mean sea level rise has grown at a rate not seen in the past 3,000 years.
These are some of the indications contained in the first of the three parts, released today, of the sixth report of the IPCC (the group of scientists experts in climate change) approved by 195 UN governments.
All the most important indicators of the components of the climate system (atmosphere, oceans, ice) are changing at a speed never seen in the last centuries and millennia, say the scientists - among them three Italian researchers from the Institute of Atmospheric Sciences and climate of the National Research Council - recalling that in 2019 anthropogenic emissions reached concentrations of 410 parts per million for co2 and 1,866 parts per billion for methane.
The global average temperature of the planet in the decade 2011-2020 was 1.09 degrees Celsius higher than that of the period 1850-1900 with a more accentuated warming on the land masses than in the ocean. The preponderant part of global warming is caused by greenhouse gas emissions deriving from human activities, reiterates the working group 1 which evaluates the new scientific knowledge that has emerged compared to the previous report of 2014. In the coming decades, experts say, an increase in climate change is expected in all regions. For cities, some aspects of climate change can be amplified. These include heat waves, floods due to heavy rainfall and rising sea levels in coastal cities.
Irreversible changes for millennia
The continuous rise in sea level is one of the phenomena of climate change already underway, "irreversible in hundreds or thousands of years", say the scientists, who note how climate change affects every area of the Earth and the entire climate system. However, they warn, strong and steady reductions in CO2 and other greenhouse gas emissions would limit climate change.
Coastal areas are expected to continue to rise in sea level throughout the 21st century which could lead to more frequent and severe flooding and coastal erosion. Extreme sea-level events that previously occurred once every 100 years could occur every year by the end of this century, scientists warn.
The report speaks of a warming that is proceeding very fast and provides new estimates on the possibilities of exceeding the global warming level of 1.5 degrees Celsius in the coming decades. Unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming to around 1.5 or even 2 degrees Celsius will be an out-of-reach goal.
The study shows that greenhouse gas emissions from human activities are responsible for about 1.1 degrees of warming over the period 1850-1900. On average over the next 20 years, global temperatures are expected to reach or exceed 1.5 degrees of warming.
"This report is an objective confirmation," said the co-chair of working group I of the IPCC, Valèrie Masson-Delmotte. "We now have a much clearer picture of the past, present and future climate, which is essential to understand where we are headed, what can be done and how we can prepare".
With 1.5 of global warming, an increase in the number of heat waves, longer warm seasons and shorter cold seasons is expected. With global warming of 2 degrees, extremes of heat would more often reach critical tolerances for agriculture and health.
The IPCC report shows that human activities still have the potential to determine the course of the future climate. The scientific evidence is clear, says the IPCC, which shows that carbon dioxide (co2) is the main driver of climate change, even as other greenhouse gases and air pollutants contribute to the climate. The reduction of co2 emissions will bring positive effects on air quality, observable on a time scale of a few years. Otherwise, the effects on the planet's temperature will only be visible after many decades. Hence the extreme urgency of timely and substantial interventions to reduce climate-altering emissions.
Five possible scenarios
In this report, five possible scenarios are simulated with the relative climate of the future that describe contexts in which there is no substantial mitigation with respect to co2 emissions, an intermediate context, with modest mitigation and contexts that describe scenarios with low co2 content with zero emissions achieved in the second half of the 21st century.
The global surface temperature is expected to continue to increase at least until the middle of the century in all emissions scenarios considered. Global warming levels of 1.5 and 2 degrees above pre-industrial levels will be surpassed by the end of the 21st century unless dramatic reductions in co2 and other greenhouse gas emissions occur in the coming decades.
In the scenario with the lowest estimated co2 emissions, i.e. with a decrease in global greenhouse gas emissions from 2020 onwards and the achievement of net co2 emissions of zero in the 2050s, global warming during the 21st century is extremely likely to remain below 2 degrees.
In scenarios with high co2 emissions, the carbon-absorbing capacity of the oceans and terrestrial ecosystems is expected to become less effective in slowing the growth rate of atmospheric co2. Many of the changes already observed in the climate system, including increased frequency and intensity of temperature extremes, heat waves, heavy rainfall, drought, Arctic sea ice loss, snowpack and permafrost, will become more intense as warming increases. global. It can be argued that every half degree of global warming causes a clearly perceptible increase in the frequency and duration of temperature extremes (heat waves), the intensity of heavy rainfall and drought in some regions of the planet.