• The Intergovernmental Panel on Climate Change's latest findings, approved by 195 member states on Friday, deals with the physical science basis of climate change and outline how humans are altering the planet.
• It provides world leaders with a gold standard summation of modern climate science ahead of U.N. climate talks, known as COP26, in early November.
• "This report is a reality check," said IPCC Working Group I Co-Chair Valérie Masson-Delmotte.

The world's leading climate scientists on Monday delivered their starkest warning yet about the deepening climate emergency, with some of the changes already set in motion thought to be "irreversible" for centuries to come.

A highly anticipated report by the U.N.'s climate panel warns that limiting global warming to close to 1.5 degrees Celsius or even 2 degrees Celsius above pre-industrial levels "will be beyond reach" in the next two decades without immediate, rapid and large-scale reductions in greenhouse gas emissions.

To be sure, the 1.5 degrees Celsius threshold is a crucial global target because beyond this level, so-called tipping points become more likely. Tipping points refer to an irreversible change in the climate system, locking in further global heating.

At 2 degrees Celsius of global warming, the report says heat extremes would often reach critical tolerance thresholds for agriculture and health.

U.N. Secretary-General, António Guterres described the report as "a code red for humanity."

"The alarm bells are deafening, and the evidence is irrefutable: greenhouse gas emissions from fossil fuel burning and deforestation are choking our planet and putting billions of people at immediate risk," Guterres said.

The Intergovernmental Panel on Climate Change's latest findings, approved by 195 member states on Friday, deals with the physical science basis of climate change and outline how humans are altering the planet. It is the first installment of four reports released under the IPCC's current assessment cycle, with subsequent reports scheduled to be published next year.

The first part of the IPCC's Sixth Assessment Report provides world leaders with a gold standard summation of modern climate science ahead of U.N. climate talks, known as COP26, in early November.

Reacting to the report's publication, U.S. Special Presidential Envoy for Climate, John Kerry said the report underscored the "overwhelming urgency of this moment." U.K. Prime Minister Boris Johnson said he hoped it could be a "wake-up call" for global leaders ahead of COP26.

Swedish climate activist Greta Thunberg said the report contained no real surprises. "We can still avoid the worst consequences, but not if we continue like today, and not without treating the crisis like a crisis."

What does the report say?

Climate scientists said it is "unequivocal" that human influence has warmed the global climate system, with observed changes already impacting every region on the planet.

Some of the changes researchers observed in the climate were described as "unprecedented," while others — such as continued sea level rise — were projected to be "irreversible over hundreds to thousands of years."

The report shows that emissions of greenhouse gases from human activities are responsible for roughly 1.1 degrees Celsius of warming since 1850-1900, and finds that averaged over the next 20 years, global temperature is expected to reach or exceed 1.5 degrees Celsius of warming.

The U.N. climate panel says "strong and sustained" reductions of carbon emissions and other greenhouses gases would limit climate change. Benefits such as improved air quality would come quickly, while it could take 20 to 30 years to see global temperatures stabilize, it adds.

The IPCC report makes clear that it is not just about temperature. It says climate change is bringing different changes in different regions — and all will increase with further global heating.

These changes include more intense rainfall and associated flooding, more intense drought in many regions, coastal areas to see continued sea level rise throughout the 21st century, the amplification of permafrost thawing, ocean acidification, among many others.

It follows a series of mind-bending extreme weather events worldwide. For instance, in just the last few weeks, floods have wreaked havoc in Europe, China and India, toxic smoke plumes have blanketed Siberia and wildfires have burned out of control in the U.S., Canada, Greece and Turkey.

Policymakers are under immense pressure to deliver on promises made as part of the Paris Agreement ahead of COP26. Yet, even as global leaders publicly acknowledge the necessity of transitioning to a low-carbon society, the world's dependency on fossil fuels is expected to get even worse.

The IPCC's Fifth Assessment Report, published in 2014, provided the key scientific input to the Paris Agreement.

Almost 200 countries ratified the Paris climate accord at COP21 in 2015, agreeing to limit the planet's temperature increase to well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C.

It remains a key focus ahead of COP26, although some climate scientists now believe that hitting this latter target is already "virtually impossible."

The IPCC has previously recognized that the necessary transition away from fossil fuels will be a huge undertaking that requires "rapid, far-reaching and unprecedented changes" across all aspects of society.

It has underscored the point that limiting global warming to 1.5 degrees Celsius "could go hand in hand with ensuring a more sustainable and equitable society," with clear benefits to both humans and natural ecosystems.

However, a U.N. analysis published earlier year found that pledges made by countries around the world to curb greenhouse gas emissions were still "very far" from the profound measures required to avoid the most devastating impacts of climate breakdown.

What is the IPCC?

The IPCC is a U.N. body of 195 member states that assesses the science related to the climate crisis.

It was founded in 1988 by the United Nations Environment Programme and the World Meteorological Organization to provide world leaders with periodic updates about the scale of the climate emergency, its implications and risks and to put forward adaptation and mitigation strategies.

It is formed of three working groups. The first, Working Group I, deals with the physical science basis of climate change. This group presented its contribution to the IPCC's Sixth Assessment Report on Monday.

Working Group II deals with impacts, adaptation and vulnerability and Working Group III assesses the mitigation of climate change. A separate task force assesses methodologies for measuring greenhouse gas emissions and removals.

Thousands of climate scientists volunteer their time to pore over the latest climate research to contribute to the work of the IPCC. The reports are drafted and reviewed at several stages and are of fundamental importance to international climate negotiations.

The Working Group II and Working Group III reports are scheduled to be finalized in February and March 2022, respectively. A concluding Synthesis Report is also due to be published next year.