Updated on March 31, 2014 at 10 a.m. ET.

The world is not ready for climate change, which poses a number of serious risks, says the planet’s leading body of climate scientists.

On Monday morning in Yokohama, Japan, the Intergovernmental Panel on Climate Change (IPCC) released a major report on the impacts of climate change, with the goal of spurring world leaders to act more decisively to reduce greenhouse gas emissions. The report warns of serious impacts from changing climate on agriculture and human civilization and argues that governments are ill prepared for its effects.

The hundreds of scientists who wrote the report argue that world leaders have only a few years left to reduce carbon emissions enough to avoid catastrophic warming. At the same time, governments must step up efforts to protect vulnerable communities from increased natural hazards associated with climate change.

"Observed impacts of climate change are widespread and consequential," the scientists of the IPCC write in the report.

The new report show 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 the impacts of climate change at the World Resources Institute in Washington, D.C.

"Crossing a Threshold"?

The new IPCC report warns that the world is close to missing a chance to limit the global warming that has occurred since the Industrial Revolution to two degrees Celsius (3.6 degrees Fahrenheit), a goal that world leaders had previously agreed was an important target. Beyond that point, "impacts will begin to be unacceptably severe," the authors wrote.

"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 four degrees Celsius, she says, as predicted by some climate models, "we would see extensive changes in agriculture." Some areas where people currently live could also be rendered uninhabitable due to extreme shifts in temperature, amount of water, or sea level.

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 says the report may be a "wake-up call, letting people know that climate change is now everywhere and that impacts are already unfolding." She hopes the report will help fill in some details and serve as a call to action for international leaders to negotiate more aggressive attempts to reduce greenhouse gas emissions.

People Aren't Ready for Climate Change

The report from Working Group II further warns: "Impacts from recent extreme climatic events, such as heat waves, droughts, floods, and wildfires, demonstrate significant vulnerability and exposure of some ecosystems and many human systems to climate variability (very high confidence). These experiences are consistent with a significant adaptation deficit in developing and developed countries for some sectors and regions."

Levin says the bottom line is that governments need to invest a lot 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."

Planners of infrastructure also need to pay more attention, she says. For example, power plants will need to have enough water to function in places that are likely to get hotter and drier.

When it comes to response to climate change, the next decade is critical and will "shape the rest of the century," says Levin. Energy companies and governments are actively planning and building the infrastructure that will be in service for decades, she notes. "Whether we pick a low-emission or high-emission pathway, we may not see changes immediately, but in terms of a century it is a drastically different world."

Still, Levin says the "window hasn't closed" on addressing climate change. There is still time to head off the most severe impacts. The new IPCC report should help show the way, she says.

In response to the report, Frances Beinecke, the president of the Natural Resources Defense Council, says, "This is an S.O.S. to the world: We can't wait any longer to sharply curb carbon pollution—the primary driver of climate change. If we don't, punishing rainfall, heat waves, scorching drought and fierce storm surges will worsen, and the toll on our health and economy will skyrocket."

Bad News for Farms

The new report specifically calls out risks to agriculture from changing climate. “There’s a lot of evidence in the scientific literature that climate extremes can impact crops,” says Noah Diffenbaugh, a climate scientist at Stanford University and one of the lead authors of the North American section of the document.

“In the U.S. we have seen acute effects of severe heat on corn, cotton, and soy yields,” he adds. “That kind of severe heat is likely to increase in response to continued global warming.”

What is the IPCC?

The United Nations-affiliated IPCC is an association of thousands of scientists from around the world that was founded in 1988. Since then it 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 prepared by three different Working Groups. Working Group I focused on the physical science behind climate change; its report was published September 27, 2013 (see the report's five key takeaways). Working Group II is releasing its report this week on the impacts of climate change and how people might adapt to them. In April, Working Group III will address how governments can work to mitigate climate change.