Most insights come as a surprise: a burst of understanding, an elegant solution to a problem. This decade's main insight in climate science was a different breed. For 40 years, researchers had wrestled with three big questions: Is the world warming? If so, are humans behind the warming? And are natural processes likely to rein it in?

Some scientists have argued from observations that global warming will alter clouds in ways that will largely counter warming by greenhouse gases. But the overwhelming majority of climate scientists sides with the models, which show clouds changing in ways that amplify warming, not dampen it. Whom to believe? To help sort it out, a climate researcher looked at the example of El Niño and La Niña, naturally occurring weather patterns that cause warming (El Niño) and cooling (La Niña) in the tropical Pacific and around the globe.

The first reliable analysis of cloud behavior over past decades suggests

In 1998, a handful of geoscientists breathed new life into a daring idea: that Earth froze over from pole to pole more than a half-billion years ago, threatening life with extinction but perhaps prodding it to greater evolutionary heights. Geoscientists report evidence that the tropics also hosted glaciers more than 100 million years before that supposed global freeze.

The two great ice sheets

The blogosphere has been having a field day with global warming's apparent decade-long stagnation. But climatologists are finding that although global warming has indeed paused, it is likely to return with a vengeance within a few years.

In the past, global climate modelers could realistically simulate either the atmosphere or the ocean, not both at once. Now a new "hybrid" model at the National Center for Atmospheric Research manages the feat. Its developers say it shows how minute changes in solar radiation can cause major climate swings such as the medieval Little Ice Age. But other researchers say such claims are premature.

If rising levels of greenhouse gases aren't pushing up global temperatures, as contrarians argue, what else could be?

After paring 23 climate models down to the best half-dozen, two researchers now say with new confidence that arctic summer ice will most likely disappear around 2037. But none of the select models predicts a tipping point--a sudden jump to an ice-free summer Arctic.

A new computer modeling study confirms that global warming is changing the salinity of seawater in the North Atlantic.