The evidence is unequivocal: Humans have warmed the planet, and every region on Earth is already affected by the climate crisis.
That's the headline message of a report released Monday by the Intergovernmental Panel on Climate Change. The document, a staggering work of international collaboration, lays out the scientific basis for the climate crisis and the unprecedented changes observed in the Earth's climate system due to human influence and activity.
It provides the most up-to-date estimates on the increasing likelihood the climate will surpass a 1.5-degree Celsius level of warming in the next decades, and -- as IPCC reports have since 1990 -- it urges immediate action to reduce greenhouse gas emissions.
The report, Climate Change 2021: The Physical Science Basis, was authored and reviewed by more than 230 leading experts in the IPCC's Working Group I. It presents a deep and robust analysis of the latest climate models, observations and scientific evidence to project a range of climate futures that could come to pass, depending on the actions taken in arresting emissions in the coming years.
"What it shows is increasing evidence that we should be concerned about climate change that's already happening, as well as the climate changes that are predicted," says Mark Howden, a climate scientist at Australian National University and contributing author on the report.
The report draws from over 14,000 references, exploring the underlying science of how increasing global temperatures affect sea level rise, glacier and ice sheet melt, reduced oxygen and increased acidification of the ocean and extreme weather events.
The report is a "reality check," according to Valérie Masson-Delmotte, a French climate scientist who co-chairs Working Group I. The likelihood of blowing past the 1.5 degree Celsius threshold outlined in the 2015 Paris Agreement is increasing and time is running out.
While the science may seem disheartening, the report offers up a small window of opportunity for policymakers to set ambitious emissions reductions targets and alter our current trajectory. "The climate picture itself is really concerning, but ultimately, we can take action, which will potentially reduce those concerns," notes Howden.
Hotter, drier, wetter and more extreme
You may be thinking "I've already heard all this before." Scientists have been banging this drum for decades and so has the IPCC. Monday's report is a critical update to a long-standing message.
"The broad message has not changed," says Lisa Alexander, a climate scientist at the University of New South Wales in Sydney and contributing author on the latest report. "But now we've got more data, more models, more details."
The IPCC was established by the United Nations Environment Programme and the World Meteorological Organization in 1988 to understand the drivers and impacts of climate change. It consists of thousands of leading experts who review and assess the scientific, social and economic aspects of the climate crisis by reviewing research and studies into climate change. Its last major assessment report was published in 2014, with the Working Group I report published a year prior.
Since then, there's been an extra eight years of data to sift through, allowing the IPCC to state, with increased confidence, how much human influence has caused global surface temperatures to rise. "In that eight years, we've seen very big changes in temperature, for example," Howden notes.
The crisis is already forcing a rethink in all aspects of human life. Just this week, the Tokyo Olympic Games reached the finish line without any major disruptions due to COVID-19 -- but a more pernicious threat shadowed the event: extreme heat.
Athletes suffered from sweltering heat and humidity, with archers collapsing and tennis players wheeled off court. Officials changed the location of the marathon from Tokyo to Sapporo, in Japan's north, back in 2019, in an effort to escape soaring temperatures in the host city -- only to find Sapporo's temperatures were so hot the race was forced to start an hour earlier.
The report focuses on some of the regional changes and extreme weather events that have already occurred, contextualizing some of the heatwaves, droughts and floods that have ravaged the globe in the last few years and using new data to strengthen their links to human activity and increasing temperatures
But increasing heat is only one aspect of the disordered climate system. In recent years, unprecedented bushfires have blazed across Australia's east coast and America's west. We've begun to see more hurricanes and cyclones along with extreme rainfall events and retreating ice, snow and permafrost cover.
With increasing global temperatures, the data shows these types of events will increase in frequency and intensity. For instance, hot temperature extremes that occurred once every 10 years between 1850 and 1900, now likely occur 2.8 times every 10 years. If emissions aren't reduced and we reach 4 degrees of warming -- as opposed to the 1.5 degrees the Paris Agreement targets -- those extremes will occur almost once a year.
The relationship between human-produced carbon emissions and global warming is near-linear, which allows scientists to predict the extent of warming for different emissions scenarios.
The report considers five scenarios or "climate futures."
Human activities, particularly emissions of carbon dioxide, have already contributed to around 1.1 degree Celsius of warming since 1850-1900. The experts write that global surface temperatures have increased faster since 1970 than in any other 50-year period in the last 2,000 years.
The 1 degree Celsius increase also forms a base level for estimating change in the lowest-emissions scenarios. Even under this scenario, temperatures will remain elevated into the next century, and the report shows the increase has already resulted in "irreversible" changes to ocean temperature and acidification, and to global ice sheet coverage. Just to prevent further damage, this scenario requires a steep and rapid drop in carbon emissions.
Other scenarios, where carbon emissions aren't immediately curtailed, project a more grim outlook.
Although 2 to 4 degrees of warming may not seem like a big increase, the impacts are far-reaching. By the end of the century, under the highest-emissions scenarios in which carbon output doubles by 2050, the ice sheets would be devastated, sea levels would increase a meter or more, and carbon sinks would become less effective at trapping carbon dioxide.
The report also shows the COVID-19 pandemic did nothing to slow the concentration of carbon dioxide in the atmosphere, though it did have detectable effects on air pollution.
The evidence has only strengthened the notion that drastic reductions in carbon emissions are the only way to avoid the most damaging climate futures.
"I think the last [report] was fairly urgent," notes Alexander. "This one's even more urgent.
"And if I'm talking to you in another six or seven years -- we have a real problem."
The IPCC's role is not to prescribe policy, but to inform decision making by governments. Howden notes that because the science and policy communities work collaboratively on this report, it's "much harder for governments not to acknowledge it and to incorporate it into decision making."
Since the beginning of this reporting cycle, the IPCC has published three special reports, examining different areas of the climate crisis. The first, published in 2018, examined how to meet a global warming target of 1.5 degrees Celsius. Two later reports, published in 2019, examined how climate change is affecting the land, the oceans and the cryosphere (the frozen parts of Earth).
Monday's report, from Working Group I, is the first of three Working Group reports due for publication in the next six months. "This report does not go into the implications for ecosystems or economies or human health and those sorts of things," says Howden. "That happens in the next report."
A second expert panel, Working Group II, is expected to release a report examining the impacts, vulnerabilities and ways to adapt in February. That report will be followed by another, published by IPCC's Working Group III, focused on how to mitigate climate change.
A final synthesis report, which comprises the analysis of all three working groups and the three special reports, will be published in September 2022, compiling all the research from AR6.