Historians say the Chesapeake Bay has changed since Capt. John Smith first landed at Jamestown four centuries ago.

And scientists say it will change again by the end of this one as a rising, warming ocean with more acidic waters carves out a different estuary and disrupts the huge diversity of marine life that depends on it.

"It will be quite a bit different," said Robert Latour, fisheries scientist at the Virginia Institute of Marine Science in Gloucester Point. "I imagine the coastline will be a little bit different — some projections put the Florida Keys underwater in 100 years, and we're pretty low-lying. In the fish populations, there will be winners and losers."

The bay is a vital space for numerous spawning species, a nursery for others and an important migratory route for countless more. Latour suspects native oysters, already decimated by overfishing and disease, could be hard-hit. And iconic blue crabs, once plentiful, could become scarce.

"We're starting to see blue crabs moving north in areas that we hadn't historically associated with them," Latour said. "So there's been a general shift in fishes, too."

But the bay is just one example, scientists say, of how a warming Earth will have real impacts at the local level. And it illustrates the message behind a new report issued this week by the U.N. Intergovernmental Panel on Climate Change. The report builds on earlier ones with more sophisticated and comprehensive science, and identifies the populations and places at risk: everyone, everywhere.

"The report concludes that people, societies and ecosystems are vulnerable around the world," Chris Field, co-chairman of the working group behind the study, said in a statement. "But with different vulnerability in different places."

According to the report's summary for policymakers, climate change isn't something coming down the pike; it's already here on every continent and in every ocean. Experts cite wildfires in the western U.S., record rains in England, extreme heat waves and droughts, changes in wind patterns, stronger coastal storms and cyclones.

And most of the increase in global average temperatures, the summary says, is "very likely" the result of the dramatic uptick in human-caused greenhouse gases that began with the Industrial Revolution.

The IPCC says the world, in many cases, is "ill-prepared for risks from a changing climate" and urges "smart actions" now to decrease those risks: curb emissions, mitigate the inevitable damage and learn to adapt to a rapidly changing world.

"Climate-change adaptation is not an exotic agenda that has never been tried," Field said. "Governments, firms and communities around the world are building experience with adaptation. This experience forms a starting point for bolder, more ambitious adaptations."

The clarion call comes hard on the heels of the report's clanging alarm bells, including:

•Concentrations of carbon dioxide and methane in the atmosphere in 2005 exceeded by far the natural range over the last 650,000 years.

•Eleven of the last 12 years (1995-2006) rank among the 12 warmest years since record-keeping began in 1850.

•More intense and longer droughts over wide areas since the 1970s, particularly in the tropics and subtropics.

•More frequent heavy rains over most land areas.

•Widespread changes in extreme temperatures over the last 50 years — with fewer cold days, cold nights and frost, and more frequent hot days, hot nights and heat waves.

•More intense tropical cyclone activity in the North Atlantic since about 1970.

As the planet warms, the report says the risks to mankind increase, including crop failures, a drop in gross domestic product, increasing poverty and hunger, violent conflicts, the loss of infrastructure and coastal communities and increases in food- and water-borne diseases.

The report also claims a "higher confidence" in its projections for continuing changes in warming, wind patterns, precipitation and ice: