Specifically, people are talking about how a severe cold snap can occur at the same time as global warming. If you haven’t seen it yet, check out the debate on the Washington Post website.  In this post, I’ll try to clear up two of the issues that emerged from that debate:

1. What does a particular cold spell say about global warming, and
2. If the recent cold spell doesn’t disprove global warming, does that also mean that other kinds of extreme weather, like heat waves, aren't caused by global warming?

Climate versus weather

All of the Post’s panelists were careful enough to explain the difference between weather and climate: Climate refers to the average weather over a long period. For the most part, they did not fall for the common mistake of interpreting a cold spell as evidence against global warming.

Here's what's been happening with the weather recently: There have indeed been below-average temperatures recently in much of the eastern U.S. and in parts of Europe, Russia, northern China, and northern India. But at the same time, there were above-average temperatures in the western U.S., eastern Canada and Greenland, some other parts of the Arctic, North Africa and Central Asia, as this map shows.

This distinct pattern of temperatures was caused by an unusually persistent version of an atmospheric flow pattern known as a “Greenland block.”  This Greenland block diverted frigid Arctic air far to the south in eastern North America and Europe. (More about it on the Weather Channel: "Why So Cold? Blame the Greenland Block.")

It’s important to look at weather events like cold snaps in context—we can have a relatively brief spell of cold weather in certain regions even while the global climate is warming.  All the evidence shows that the world overall has been warming over the past several decades. (See a chart in a post on this same topic by Lisa Moore in 2008.)

So how do we know if the climate is warming? We look at a wide range of long-term trends. Along with rising air and ocean temperatures, the other signs of a warming climate include rising sea level, retreat of glaciers in most regions, rapid shrinkage of summer sea ice in the Arctic, and shifts in species distributions and seasonal behavior.

Global warming does cause more extreme weather

Although the Post’s panelists were accurate on the first issue, there could have been more discussion on the fact that global warming does have an effect on some kinds of extreme weather. One of the panelists even claimed that extreme events like heat waves cannot be used as evidence of global warming. That is wrong, so let's look at how the frequency and intensity of certain extreme weather events are expected to increase under global warming.

Records indicate that there has already been an increase in intensity and frequency of heat waves and heavy rainfall in many parts of the world over the past several decades.  (See “Frequently Asked Question” number 3.3 excerpted from the 2007 IPCC report [PDF].)  Why? Global warming drives a rise in average temperature and atmospheric moisture, promoting more heat waves and torrential downpours. On top of that, changes in atmospheric circulation patterns caused by global warming are also thought to contribute to stronger heat waves.

There will still be variations from year to year, but on average, these extreme events will increase over time as the Earth warms.  On the other hand, extremely cold temperatures are becoming less common — but can still occur — as heat builds up in the climate system.

Of course, individual weather events should not be blamed on global warming, just as an individual cold snap doesn’t disprove global warming. EDF has been careful not to attribute individual events to global warming.  Instead, we point to examples of what we expect to see more and more of in the future if we don’t fight global warming.

And with the trends in extreme weather we’re already seeing, that future ain’t lookin’ pretty.

Here are some of the "business-as-usual" projections that my colleagues and I find most striking and disturbing:

You think August is hot now?

By the end of this century, we could be in for much more severe summers all across the country.

• If you live in New Hampshire, summer could feel like it does today in North Carolina (p.107).
• If you live in Michigan, brace yourself for summers that feel like today's summers in Oklahoma (p 117).
• And if you live in Texas, you now experience 10 to 20 days a year over 100 °F. By the last two decades of this century, look for 100 such days – that's more than three months (p. 90).
• In 1995, Chicago suffered a heat wave that killed more than 700 people. Chicagoans could experience that kind of relentless heat up to three times a year (p. 117).
• The Southwest, including cities like Los Angeles and Phoenix, will face worse and more frequent droughts, as spring rains decline by as much as half, snowpacks shrink and melt earlier, and water evaporates more rapidly (p. 129-130).

People who live on the coasts could be a lot closer to the shore

Sea level is projected to rise up to 3 to 4 feet. Here's what that means for various parts of the country:

• Portions of New York City and Boston could be regularly flooded by storms and even high tides (p. 150).
• On the Gulf Coast, approximately 2,400 miles of roads and 250 miles of freight rails are likely to be permanently flooded (p. 62). This area is home to seven of the nation's ten largest ports and much of our oil and gas industry.
• Some coastal freshwater sources will be contaminated with saltwater, meaning we can no longer use them for drinking water without expensive desalinization (p. 47)

Your grandchildren will miss out on local icons and specialties

The foods and activities that define different parts of the country are changing.

• Some western ski resorts could face a 90 percent decrease in snowpack, making the country's most iconic ski locations just shades of what they are today (p. 133).
• Thanksgiving might no longer include cranberries produced in the Northeast's cranberry bogs (p. 73).
• In the Northwest, salmon will be driven out of about one-third of their habitat. We could start to see the changes in the next ten years (p. 137).

This very thorough scientific report paints a bleak picture of what life will be like in this country if we let pollution continue at today's rate. The report's good news is that if we act now, we can avoid the most severe consequences. But the more sobering news is that even if we cut emissions aggressively, not everything in this report can be avoided. This is a first step toward understanding how to prepare for the coming changes.

The American Clean Energy and Security Act, which would take us off the "business-as-usual" path, will come in front of the U.S. House for a vote in a matter of days. This report gives our leaders yet another reason to do the right thing for our country's future.

Hurricane Ike storm surge hits Jetty East, by Flickr user eh3k.

For some, it’s hard to care about global warming because its impacts on everyday life aren’t obvious. It’s too abstract. It’s not tangible. It’s too wonky. It’s just not real enough.

Well, what if you learned that global warming could literally push you out of your home? Is that real enough for you?

Texas cities Houston and Galveston just got a wake-up call – conservative estimates of sea level rise due to climate change will displace 78 percent of households over the next 100 years in Galveston County alone, according to a new study that EDF and the British Consulate-General commissioned from the Harte Research Institute at Texas A&M Corpus Christi.

And that's the conservative estimate. In a business-as-usual scenario, Galveston-area sea levels could rise as much as 1.5 meters in the next 100 years, which could displace more than 100,000 households and create more than $12 billion in infrastructure losses for Houston and Galveston.  Rising sea levels will also damage at least 23 public facilities, industrial sites and water treatments plants, begging many questions about where to move or how to protect these sites. (See a related post on sea level rise.)

For cities like Galveston, which is still rebuilding from Hurricane Ike, these findings reinforce the notion that planning for how to adapt to climate changes or mitigate their effects is critical. Current discussions have centered around building a large, expensive and likely ineffective “Ike Dike” to protect the city from future storm surges, though there are likely more realistic adaptation measures that will make people safer quicker. Passing federal climate legislation and ramping up local municipal energy efficiency are surely the most effective ways to begin reducing the rate of sea level rise.

So, for all of you out there who don’t think global warming could happen to you, take a hard look at the harsh reality that these coastal communities face.

Check out our hurricane and clilmate change overview for the straight facts from EDF climate scientists. And Peter Black shares eye-opening maps over on ClimateAtlas.

This post is by Sheryl Canter, an online writer and editorial manager at Environmental Defense Fund.

Yesterday, an unusually ferocious winter tornado system killed at least 55 people in five Southern states, and destroyed over a thousand homes and businesses. Usually tornadoes touch down for 20 minutes then come back up, but these were rare "long-track" tornadoes that stayed on the ground for 30 to 50 miles.

Lots of recent research indicates that global warming increases the intensity of hurricanes. Could it also affect tornadoes? It's too early to tell, but scientists are starting to look.

The relationship between global warming and tornadoes is a very new area of research. The IPCC's Fourth Assessment Report says that there is "insufficient evidence" of any changes in "small scale phenomena such as tornadoes, hail, lightning and dust-storms." So there's no trend, but a recent study by NASA found that "the most violent severe storms and tornadoes may become more common as Earth’s climate warms."

Tornadoes are formed when warm and cold air come together, and according to the Weather Underground, yesterday's tornado outbreak was fueled by unusually warm weather. That's not definitive, of course, since no single weather event can be ascribed to global warming, but it makes you wonder. I'm glad scientists are starting to investigate.

Last week, scientists published a study about global warming and hurricanes in the U.S. The authors found a weak downward trend in the number of landfalling U.S. hurricanes, but concluded that future trends will depend on the spatial distribution of ocean warming.

In its coverage of the study, the Miami Herald said that "global warming actually is diminishing the number of hurricanes that strike Florida and the rest of the United States – and the phenomenon is likely to continue." This is not exactly what the study said. Here's the full scoop.

Atlantic hurricane tracks, 1980-2005.

When it comes to hurricanes, there are two important concerns: storm intensity and storm frequency. There is much solid research establishing a relationship between global warming and increased storm intensity, and this recent study does not contradict it. This new study is about storm frequency, not storm intensity.

The authors looked at the number of hurricanes making landfall in the U.S. since 1854 and found a slight decrease over time. They suggest that this weak trend is from increased "wind shear" in the Atlantic Ocean caused by warming in the tropical Pacific and Indian Oceans. Wind shear occurs when wind speed or direction changes with altitude. This can inhibit storm development by "shearing" off the top of a nascent storm.

There has been other research suggesting that global warming will increase Atlantic wind shear, thus decreasing storm frequency. But with warmer oceans, when storms do come, they will tend to be fierce. In discussing another paper about wind shear, climatologists at the RealClimate blog said:

While increases in wind shear could offset the impact of tropical temperatures in some – maybe even the majority – of storm seasons, one might worry about what happens during those seasons where there is anomalously low shear (e.g., a very strong La Niña event). The warm ocean will still be sitting there, waiting to produce tropical cyclones and Hurricanes – and the prospects for destructive Hurricane activity during those seasons could be especially grim.

Wind shear will not reliably protect us from landfalling hurricanes.

The authors of the study found a significant relationship between rising global sea surface temperature (SST) and wind shear, and a significant relationship between wind shear and U.S. landfalling hurricanes. But the downward trend in the number of U.S. landfalling hurricanes since 1854 was not statistically significant. Essentially the line was flat.

Moreover, the authors don't say that the number of U.S. landfalls will go down in the future. They say that future trends will depend on the spatial distribution of warming and other factors: "…if the effects of warmings in the tropical Pacific and Indian Oceans cannot overcome that of Atlantic warming, global warming may favor landfall incidence for the United States."

If you'd like to learn more about hurricanes and global warming, visit our Extreme Weather site. It includes summaries of the latest research and maps of storm surge risks for various states. Check it out!

It's been raining a lot in some parts of the world. In Northern India, Bangladesh, and Nepal, the worst rains in decades have devastated tea and rice crops, killed at least 3000 to 4000 people, and affected tens of millions of people since June. Great tracts of land are under water. Crop land has been destroyed; people are hungry. And the weather forecast says rain.

Click image to view original page.

Torrential rains also have caused severe flooding in Korea, Uganda, and Sudan. In North Korea, over 600 people are dead or missing, 170,000 people displaced, and almost 1 million people affected. In Sudan, 200,000 are homeless from floods. India, itself reeling, sent $250,000 in aid to flood-ravaged Uganda.

Is this all from global warming?

I asked James Wang, Ph.D., a climate scientist at Environmental Defense, and he said, "This is definitely consistent with global warming predictions. You can never be sure about a specific weather event, but when extreme events happen simultaneously in many parts of the world and long-standing records for rainfall are broken, odds are that global warming is an important factor."

The crises in Asia and Africa show that we can't just "adapt" and take our time to find solutions, as Cool It author Bjorn Lomborg recommends. There's already significant human suffering from global warming, especially in developing countries. Still, it's not too late to act, and in fact, we must act. If we don't, the consequences will only get worse.

A week or so ago I wrote about all the recent extreme weather in the United States. Well, it's not just happening here. Great Britain is being hit by the worst flooding in decades, and it keeps on raining. A heatwave blanketing south-eastern Europe has killed hundreds of people, and out-of-control wildfires are burning across Greece.

Is this global warming? It's impossible to say for sure what causes a particular weather event, but global warming does make extreme weather events more likely. Whether or not the events in Europe are due to global warming, I do think the Europeans are, unfortunately, getting a taste of things to come. For more, visit our Web page on global warming and extreme weather.

From NOAA's National Climatic Data Center

The graph above shows drought conditions in the U.S. during June 2007 using the "Drought Severity Palmer Index" – the more positive the index (green areas), the wetter the soil, and the more negative the index (red areas), the drier the soil.

As you can see, two large regions of the U.S. – the southeast and the southwest – are experiencing extreme drought. In Georgia and Florida, dry conditions caused failing crops and water shortage alerts. Lake Okeechobee in South Florida, second only to Lake Michigan as the largest fresh water lake in the U.S., and a source of water for millions of people and the Everglades, reached its lowest level ever. Levels were so low that archeological artifacts were revealed on the exposed lake bottom.

The region was also hit with wildfires reported as the most extensive since 1898. In Georgia and Florida alone, more than 1 million acres have burned so far in 2007. At the Florida Climate Summit in Miami last week, I heard Governor Crist report that this year there were wildfires in all but 3 of the more than 90 counties in the state.

In the southwest, California, Nevada, Colorado, and Utah were hit with sweltering heat. Record-breaking or near record-breaking high temperatures were recorded in Reno, Las Vegas, Boise, and Spokane. In Phoenix, Las Vegas, and other cities, temperatures hovered near 110^oF or higher.

Meanwhile, the central part of the U.S. – especially Texas, Oklahoma, and Kansas – is awash in water. You might think these folks would appreciate moisture after last year's severe drought with widespread wildfires and crop failures. But not this much moisture. The floods produced by near-record rains have cost lives, destroyed property, and contaminated waters and soils.

Is this extreme weather due to global warming? It's impossible to attribute a specific meteorological episode to a climate trend, but global warming does make extreme weather events more the norm than the exception. And certainly the weather the past few months has been extreme.

The hurricane season began on June 1, and we are all waiting with some dread to see what this year will bring. Will it be a relatively mild season like last year, a devastating one like 2005, or something in between? An ominous sign is that we have already seen two tropical storms in the region: Andrea, which formed almost a month before the season began, and Barry, which formed on the first day of the season.

Last week I read that Oman was hit by Tropical Cyclone Gonu. ("Hurricane", "typhoon", and "cyclone" are all names for the same thing.) I know that Asia and Australia are regularly hit by tropical storms, but the Arabian Peninsula? Is that normal? I didn't know, so I decided to do a little investigating. Here's what I learned.

Tropical storms do form over the Indian Ocean, although not as commonly as they do over the North Atlantic and Western Pacific. According to NASA, these storms form most often off the east coast of India, and make landfall over the Indian subcontinent. But Gonu formed over the Arabian Sea, to the west of India. Typically the storms that form over the Arabian Sea are weak and unremarkable.

Gonu was anything but unremarkable. It appears to have been the most intense storm on record for the Arabian Peninsula. On June 4, 2007, Gonu had winds exceeded 131 mph, giving it Category 4 status, and was headed for the coast of Oman and the Strait of Hormuz.

Global warming is known to increase the likelihood of extreme weather events. Was this particular cyclone a result of global warming? No one can say. But with global warming, we are likely to see more and more unusually intense storms.

The storm in Oman was potentially bad news for gasoline prices here in the U.S., since about 20 percent of all oil shipments pass through the Strait. By the time Gonu reached the coast of Oman, it had weakened to a Category 1 storm. There was no major disruption to the flow of oil, but the damage was significant, with an estimated 49 dead and widespread flooding. Gonu then headed north to Iran where it killed 12 people, but again caused no major damage to oil supplies.

The tale of Gonu reminded me of a couple of things:

• We tend to focus on hurricanes that form in our part of the world, but they also are found elsewhere. While we had a mild hurricane season last year, China and Australia were pummeled.
• Events in one part of the world can have local repercussions. You might think, at first glance, that a tropical storm over the Arabian Peninsula would not be relevant to life in the U.S. But we live in a global community. A devastating storm in that part of the world can have a direct impact on how much we pay at the gas pump.

Global warming means change for all of us, regardless of where the storm hits.