This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense Fund.

There's an email making the rounds lately - an invitation to take a Global Warming Test. It starts off reasonably enough (yes, global warming is real), but then quickly devolves into ridiculousness - ridiculousness presented in a way that looks misleadingly "scientific".

For those interested in truth, here are the real answers to the Global Warming Test.

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense Fund.

According to NOAA's National Climatic Data Center, this past winter was the coolest since 2001. A single season can't determine a long-term trend any more than a single month can (see my previous post, "Did Global Warming Stop in January?"). But the recent cooler temperatures do offer an opportunity to talk about La Niña - a climate pattern that causes short-term cooling.

This winter's La Niña is the strongest (coldest) since 1989, so we'd expect the weather to be cooler than usual. But even so, the cooling didn't come close to offsetting the warming of the past 50-100 years. As you can see in the graph below, the cooling barely takes us back to 2001.

Data source: NASA. Each dot is a three-month period (season).

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This post is by Sheryl Canter, an Online Writer and Editorial Manager at the Environmental Defense Fund.

On Monday, the Washington Post reported that new studies say carbon output must be near zero to avert danger. They referred to two recent studies, one published in Geophysical Research Letters and one in the journal Global Biogeochemical Cycles.

I asked the scientists here at EDF about this. It turns out what's missing from the Washington Post article is the time frame. Yes, we have to bring emissions to near zero, but it doesn't have to be tomorrow. And moreover, this isn't news!

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

January 2008 was the coldest month in several years, according to the scientific groups that track global temperature. Is it true, as DailyTech concludes, that January's cold "wipes out a century of warming"? Has global warming stopped?

In a word, no. A single month does not make for a climate trend. Let's look at the data.

Source: Raw data from the U.K.'s Met Office Hadley Centre (see description of data).

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

In a comment on my Global Warming Crib Sheet, johnmashey asked if I could say something about the lifetimes of greenhouse gases - that is, how long different greenhouse gases stay in the atmosphere. Great idea! Here's a whole post on it.

Many people don't realize that the greenhouse gases we emit can stay in the atmosphere for decades, centuries or even millennia. That's why it's so important that we cap emissions as soon as possible.

Here's a table showing a selection of greenhouse gases, their global warming potential (GWP), and their lifetimes:

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

The science behind global warming is, well, science, and it can get pretty technical. By how many degrees has the globe already warmed? How much more can it warm before we're in trouble? How much carbon dioxide is in the air now, and how much more can we afford to emit before risking climate catastrophe? Which are the most important greenhouse gases? And what do all those funny abbreviations mean?

Below you will find a handy crib sheet that gives you all these numbers and more.

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

The term "tipping point" refers to a critical threshold at which a small change can qualitatively alter the state of a system. For example, when temperature reaches 32°F, ice changes into water. There also are "tipping points" in global warming. The best known is the Greenland Ice Sheet, which could begin a slow, irreversible meltdown if global temperature passes a certain threshold.

Last week, climate researchers published a paper that examines Earth systems in danger of passing tipping points due to human activity. They call these "tipping elements", and highlight nine such systems from around the world. They say the greatest threat is to the Arctic, followed by the Greenland Ice Sheet. Here's the list.

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This post is by Sheryl Canter, an Online Writer and Editorial Manager at Environmental Defense.

The U.S. Environmental Protection Agency (EPA) has come out with a neat new calculator that can make sense of all those arbitrary-sounding greenhouse gas numbers.

What does a metric ton of carbon dioxide look like? The calculator shows a dozen different equivalencies, such as 114 gallons of consumed gasoline, 2.3 barrels of oil, or 0.01 acres of forest preserved from deforestation. And it's not limited to metric tons or CO₂. You also can try different amounts and different greenhouse gases.

The EPA developed the calculator to help people in communicating about reduction targets, but it's useful to anyone trying to get a handle on the numbers.

This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

The results are in for 2007, and it was a warm one. How warm? It depends who you ask. According to NASA, 2007 was the second warmest year on record. NOAA's analysis put it in fifth place. The University of East Anglia's Climatic Research Unit (CRU) ranked it eighth.

The results differ because it's not as straightforward as you might think to take the temperature of the globe. There are many subtleties to consider.

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This post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense.

All year long we've been monitoring developments in climate science, and posting about the important new developments. I thought now would be a good time to look back over 2007 and summarize what we've learned.

Here are ten noteworthy science stories we covered in 2007:

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