The world is not ready for the impacts of climate change, including more extreme weather and the likelihood that populated parts of the planet could be rendered uninhabitable, says the planet's leading body of climate scientists in a major new UN report.

The 772 scientists who wrote and edited the report argue that world leaders have only a few years left to reduce carbon emissions enough to avoid catastrophic warming, which would produce significant sea level rise and large-scale shifts in temperatures that would dramatically disrupt human life and natural ecosystems.

"Observed impacts of climate change are widespread and consequential," according to the report, which is from the UN's Intergovernmental Panel on Climate Change (IPCC) and was released Monday morning in Yokohama, Japan.

The report uses stronger language around current impacts of climate change than past IPCC releases.

It warns that the impacts from changing climate are already happening, calling out "high risk levels" for spread of disease in Africa; property loss and mortality due to wildfires in North America; and decreased food production and food quality in South America. (See video: "Global Warming 101.")

The report also warns of more dire consequences to come and says governments are ill-prepared for the effects.

It shows that "today's choices are going to significantly affect the risk that climate change will pose for the rest of the century," says Kelly Levin, a scientist who studies climate change impacts at the World Resources Institute in Washington, D.C.

Frances Beinecke, the president of the Natural Resources Defense Council, called the report an "S.O.S. to the world."

"Crossing a Threshold"

The new report, from a body known as the UN panel's Working Group II, warns that the world is close to missing a chance to limit the global warming that has occurred since the Industrial Revolution.

World leaders had previously agreed on a target of limiting warming to 2 degrees Celsius (3.6 degrees Fahrenheit).

Beyond that point, "impacts will begin to be unacceptably severe," the authors wrote. (Quiz: What You Don't Know About Climate Science.)

"There is potential for crossing a threshold that leads to large system changes, and that's a very unknown world that has severe consequences," Levin says.

If the warming were to go beyond 6-to-7 degrees Fahrenheit (about four degrees Celsius), she says, as predicted by some climate models, "we would see extensive changes in agriculture."

Even at the lower end of predictions, the report warns: "Climate change will lead to increased frequency, intensity and/or duration of extreme weather events such as heavy rainfall, warm spells and heat events, drought, intense storm surges and associated sea-level rise."

Levin hopes the report will spur international leaders to negotiate more aggressive attempts to reduce greenhouse gas emissions. (How does the greenhouse effect work?)

People Get Ready

Levin says the IPCC report's bottom line is that governments need to invest more in planning for the impacts of climate change. Communities that are already marginalized, she says, including the urban poor, are most at risk.

Some communities should be moved to less risky areas, and support services need to be bolstered, she says: "We need more fast-acting institutions and early-warning systems. We are already committed to significant warming, so adaptation is a great necessity."

Energy companies and governments are actively planning and building the infrastructure that will be in service for decades, she notes. As climate change continues, power plants will need to have enough water for cooling their systems in places that are likely to get hotter and drier.

"Whether we pick a low-emission or high-emission pathway, we may not see changes immediately," Levin says. "But in terms of a century it is a drastically different world."

The new report specifically calls out risks to agriculture.

"In the U.S. we have seen acute effects of severe heat on corn, cotton, and soy yields," says Noah Diffenbaugh, a climate scientist at Stanford University and one of the lead authors of the North American section of the document.

"That kind of severe heat is likely to increase in response to continued global warming," he says.

Understanding the IPCC

The IPCC was founded in 1988 and has released a report on the current state of scientific knowledge about climate change roughly every five years.

The new Fifth Assessment Report (AR5) updates the science since the last report was issued in 2007.

The massive report, running hundreds of pages, is being released in three sections, each prepared by a different working group.

Working Group I focused on the physical science behind climate change; its report was published last September (see the report's five key takeaways).

In April, Working Group III will address how governments can work to mitigate climate change.