The scrolling text at the end says this:

Alaska's northern coastline is eroding at rates as high as 30 meters (100 feet) per year. Climate change may be large responsible. As Arctic sea ice seasonally recedes, large regions of ocean become exposed to the sun's energy. The seas transfer this heat to the shoreline, melting the once permanently frozen land. Meanwhile, longer open-water periods allow storms to batter the weakening coast. These feedbacks may intensify as sea ice continues to shrink. Learn more at cires.colorado.edu.

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

A few weeks ago, I returned from a voyage called the Arctic Expedition for Climate Action. Sponsored by the Aspen Institute, the National Geographic Society, and Lindblad Expeditions, our group [PDF] included over 100 business leaders, scientists, environmentalists, journalists, politicians, religious leaders, and community activists.

In a word, it was sensational. We set out by ship from Svalbard – almost the closest land to the North Pole, and a three hour plane flight from Oslo, Norway. This is by far the closest to the North Pole I've ever been. My prior trips to the north shore of Alaska at Prudhoe Bay and the north coast of the Arctic National Wildlife Refuge were much further south.

There are no trees, bushes, or even grasses on Svalbard – just lichens, mosses, and very low-to-the-ground flowering plants. Plants raise their heads at their peril with Arctic winds and temperatures. We were there at the optimum time for flowering and saw many plants in bloom, including the Svalbard poppy and the compass plant. The top few inches of the tundra are unfrozen in July, though I don't think temperatures ever reached 40 degrees during our stay. The ground was a wonderful spongy carpet of brilliant colors.

National Geographic took video and photographs non-stop, including video from an unmanned submersible which showed the rich diversity of marine life in the Arctic. One of the guides showed us a particularly striking video clip. Someone took a match to melted permafrost, and it exploded like a blow torch or gas oven. Methane from melting permafrost is contributing dangerously to global warming.

We were treated to several polar bears sightings – some on icebergs near the ship. One, having just gorged on a seal, lay sprawled out with his bear belly hanging off him and onto the ice, in a food coma and seemingly oblivious to the ship's presence. We spotted as many as 10 polar bears in one sighting.

To hunt for food, polar bears need summer sea ice, which is vanishing. We heard about one starving mother bear who weighed less than 200 pounds at one point. Luckily, when caught and weighed some months later, she was back up to 1000 pounds.

We saw many other animals as well – many caribou (also called reindeer), Atlantic walrus, a very rare sea bird feeding on a dead seal, and cliffs with thousands of sea birds that seemed to all be talking in unison.

The sun never set on our entire exploration, which made for many late night discussions – helped along by a midnight feeding provided by the crew.

On the second day of the trip we toured an old whaling site with gigantic Bowhead whale bones. Bowheads have been all but wiped out in this area by commercial whaling. To me, the bones were a grim reminder that we humans are capable of awful atrocities toward our natural world that impoverish our lives and threaten our own survival.

On another day we hiked with Jimmy and Rosalynn Carter. Jimmy's perseverance in climbing a hill at age 81 was remarkable, but outdone by our two kayaking outings – especially the second one. On our first outing, the water was filled with icebergs, but it was a lovely, sunny day. The second outing was on an overcast, 32-degree day, kayaking on ice water 200 yards from huge glacier, with frozen wind blowing off the glacier. They are an amazing couple.

While kayaking, my wife Laurie and I passed by an iceberg with bearded seal lying on it. Later a bearded seal swam right up to the kayak. When its large dark eyes looked up into my eyes, I felt a deep desire to be part of efforts to ensure these creatures aren't decimated by climate change and the other all-too-prevalent threats.

This month, while Arctic sea ice hits its annual wintertime high (such as it is – see last week's post), Antarctic sea ice reaches its summertime low.

We've already posted about the British Antarctic Survey's report of a vast ice berg on the verge of breaking off the Wilkins Ice Shelf. Here's more on what's happening at the South Pole from NASA's recent briefing on polar sea ice.

Although the Arctic and Antarctic are both at the Earth's poles, they're not mirror images of each other. There are some fundamental differences between them. Antarctica is a land mass surrounded by an ocean, while the Arctic is basically an ocean surrounded by land.

Unlike the Arctic, the Antarctic typically has little perennial sea ice. There are two main reasons:

• Because there are no surrounding continents, Antarctic sea ice can float northward into warmer waters where it melts.
• Because it's at a lower latitude, Antarctic sea ice receives more direct sunlight and heat in summer.

Almost all the sea ice that forms during the winter melts during the summer.

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Also unlike the Arctic, which is one of the fastest-warming places on Earth, surface measurements and satellite data in Antarctica haven't revealed overall trends in temperature or sea ice area. Warming and sea ice loss in some areas – notably the Antarctic Peninsula, where the iceberg is breaking from Wilkins Ice Shelf – have been balanced by little temperature change or even cooling and sea ice gain in other areas.

Still, that doesn't prove there's no warming trend in Antarctica. Satellite data has only been available since the 1970s. Earlier observations from whaling ships suggest that there was a greater sea ice area before satellite observations were available.

If Antarctica isn't warming – or if it's warming at a slower rate – it may be due to the atmospheric vortex circulation that surrounds it (from being a land mass centered at a pole and surrounded by ocean). This tends to hold in cold air. But that's just one hypothesis that scientists are exploring.

Last summer we saw record-shattering shrinkage of Arctic sea ice caused, in part, by human-induced global warming. Last week I listened in on a NASA briefing on polar sea ice, and this year looks no better.

This winter was relatively cold due to a strong La Niña, so the Arctic saw a modest increase in overall sea ice (slightly above the record low of 2005-2006, but still below the long-term average). However, the older, thicker ice that lasts through the summer has declined sharply, and this is very worrying.

There are two kinds of sea ice:

• Perennial or multiyear – ice that has survived at least one summer melt season.
• Seasonal or first-year - ice that typically forms each winter and disappears the following summer.

Seasonal ice is thinner than perennial – at most four feet thick versus 15 feet for perennial ice – so it breaks up and melts more easily.

Most of the ice cover in the Arctic this year is seasonal. Scientist Walt Meier described this winter's ice as "a façade" like buildings in a movie set that look solid but are actually empty shells.

Perennial ice used to cover 50 to 60 percent of the Arctic. This year it covers less than 30 percent. Very old ice (existing for 6+ years) has shrunk from over 20 percent in the late 1980s to just 6 percent this winter.

Vast regions that used to be covered by multiyear ice are now covered by first-year ice (shown in red below):

Left: February distribution of Arctic ice by age, 1985-2000 average. Right: February 2008 distribution of Arctic ice by age. Source: National Snow and Ice Data Centre.

The amount of perennial sea ice in the Arctic is at a record low. One contributing factor is last summer's record melt season. Some of the remaining perennial ice was pushed out of the Arctic by ocean currents, resulting in further loss.

The high proportion of seasonal ice means the ice pack is especially vulnerable to melting during the summer season – very bad news for polar bears and traditional hunting societies.

A huge Antarctic ice berg – seven times the size of Manhattan – is close to breaking off, supported only by a thin strip of ice hanging between two islands.

Part of the Wilkins Ice Shelf, the berg was captured in satellite and video images by British Antarctic Survey (BAS), which said, "It is another identifiable impact of climate change on the Antarctic environment."

On January 13, Nature Geoscience published an article that reports large increases in ice loss from West Antarctica over the past 10 years. It's a sobering result that's in line with earlier, independent studies.

But then why do some people say that Antarctic ice is growing?

There are three regions of Antarctic ice: the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet (WAIS), and the Antarctic Peninsula. Most studies indicate that the EAIS is unchanged or growing slightly, while the WAIS and the Peninsula are losing ice.

Measurements of the entire ice sheet from 1993 to 2003 have ranged from 50 gigatons (Gt) growth per year to 200 Gt loss per year, according to the IPCC Fourth Assessment Report (IPCC AR4). But overall, the trend is towards loss of ice. Even with the higher uncertainty for Antarctica compared to Greenland, the AR4 concludes that "losses from… Antarctica have very likely contributed to sea level rise".

So why do some people imply the opposite?

The Inhofe report quotes Duncan Wingham as saying: "72% of the ice sheet covering the entire land mass of Antarctica is growing." It's hard to know for sure, but if Wingham was referring to the EAIS, he’s correct that some older studies inferred increased snow accumulation there. What's easy to miss is that Wingham said parts of the Antarctic ice sheet are growing rather than shrinking.

More than that, the most recent studies have found no evidence of increased
snow accumulation on the EAIS overall.

And then there is the question of model projections versus observations. The IPCC AR4 says:

Current global model studies project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall.

This means the models made two predictions: more snow and little melt. But the IPCC also points out that recent observations don't match the model projections. Antarctic ice is, in fact, shrinking. The ice sheet models were wrong on both counts.

Researchers have been expecting Arctic ice to melt and shrink, but not this fast! The National Snow and Ice Data Center, part of the University of Colorado at Boulder, reports that Arctic sea ice has shrunk to a 29-year low, significantly below the previous record low set in 2005.

In September 2007, the Northwest Passage, a direct shipping route from Europe to Asia that explorers sought for centuries, was ice-free for the first time since satellite records began.

As of September 16, sea ice extent was 1,590,000 square miles, surpassing the previous one-day record of September 20–21, 2005, by more than 461,000 square miles – roughly the size of California and Texas combined.

Arctic ice extent from satellite photos. The picture on the left was taken in September 1979; the picture on the right was taken on September 9, 2007. Source: National Snow and Ice Data Center.

NASA scientists are struggling to explain the sudden drop. Arctic ice cover can vary quite a bit from year to year, and this year's weather did encourage melting. There were unusually clear skies, warm winds from the south, and winds and currents moved ice out of the Arctic Ocean next to Greenland. But natural variations don't account for the huge magnitude of the drop. Global warming is the primary cause of this year’s record low sea ice, as well as the significant decreases over the past few decades – that much is clear. But scientists still aren't sure why the ice cover is shrinking so much faster than predicted.

A frame from an Environmental Defense ad campaign about the danger of unchecked global warming.

"Most Polar Bears Gone by 2050". You may have seen that headline in the news this week. The study behind this depressing conclusion could land polar bears on the list of threatened species under the Endangered Species Act.

Polar bears depend on the sea – and sea ice – for food. Most often they dine on ringed seals, which they catch by hanging out on sea ice next to seals' breathing holes. Without sea ice, they can't get enough food to survive. Unfortunately, summer sea ice has been declining rapidly, raising fears for polar bear survival.

In response to a lawsuit from a number of environmental groups, the U.S. Fish and Wildlife Service recently proposed protecting polar bears under the Endangered Species Act. As part of the decision-making process, the Department of the Interior commissioned a study of polar bears, and how global warming is affecting them. The resulting nine-part report evaluates the distribution and abundance of polar bears in each of four different eco-regions.

Biologists have been studying polar bear populations for years, so they have a good understanding of how sea ice dynamics – particularly the length of the ice-free season – affect polar bear survival. The authors combined this information with climate model simulations of sea ice extent under a business-as-usual scenario of greenhouse gas emissions. The results were sobering.

Polar bear populations in the Western Hudson Bay and Southern Beaufort Sea are declining, and this trend is linked to disappearing sea ice. Polar bears in the Hudson Bay and northern Russia could disappear by mid-century. Polar bears of northern and eastern Greenland may hang on for 75 years. In the archipelagos between northeastern Canada and northwestern Greenland, polar bears are likely to survive through the end of the century, though in smaller numbers.

And these conclusions are conservative. Sea ice models have generally underestimated sea ice loss.

This coming January, the Fish and Wildlife Service is expected to make a recommendation to the Secretary of the Interior on whether or not to add polar bears to the list of threatened species. The legal outcome is a few months away, but in the meantime this comprehensive scientific report hammers home the message that global warming jeopardizes Earth's stunning biodiversity, including some of its most loved, iconic species.

This is deeply alarming, but not a reason to give up. Remember, these results are for a business-as-usual scenario, and scientists have shown that bold action today can minimize global warming. This report is yet more motivation to get going in the fight against global warming.

If you had any doubts that the globe is warming and the Arctic is melting, this month's flag incident should put them to rest. A Russian submarine dove to the bottom of the Arctic Ocean to plant a flag in the seabed. Why? There's oil and gas in the Arctic seabed, which is now becoming accessible due to global warming.

Russia is not the only country vying for Arctic rights. Canada and Denmark are arguing about rights to the Northwest Passage, and the U.S. is getting into the act as well. But unfortunately, more than just oil and gas will be exposed as the Arctic melts.

Frozen soils in places like Siberia contain vast amounts of organic matter, built up as plants grew, died, and then froze instead of decomposing. If the permafrost thaws, the organic matter will start to decompose, releasing huge amounts of carbon dioxide and methane. The feedback effect from the additional greenhouse gases would greatly accelerate global warming.

We're at the cusp of a dangerous tipping point.

Most people don't realize it, but "black carbon" or soot – the same stuff that dirties up chimney flues and car mufflers – is a significant contributor to global warming. Today's best estimates place it next in line behind the greenhouse gases CO₂ and methane (see Table 2.13 in the IPCC report).

Soot particle under a microscope. Credit: D.M. Smith, University of Denver.

Greenhouse gases and atmospheric soot heat the Earth in different ways:

Greenhouse Gases. The Sun radiates light in a wide spectrum of wavelengths, mostly visible, some not. Our atmosphere is transparent to visible light. Some visible light hits the Earth, is absorbed by land, oceans, and vegetation, transformed into heat, and re-radiated as infrared light. Our blanket of greenhouse gases prevents some of the infrared light from radiating back out to space – and it's a good thing, or the planet would be too cold to support life. The problem of global warming arises when the blanket of greenhouse gases becomes too thick, as it is today.

Atmospheric Soot. Not all the visible light that hits the Earth is absorbed. Some bounces off the Earth and is reflected back out to space. This happens especially at the poles, because snow and ice are highly reflective. Atmospheric soot absorbs visible light from the Sun that otherwise would be reflected back out to space. That is, it reduces the Earth's reflectivity, or "albedo". Atmospheric soot also contributes to global dimming, which cools the Earth by screening out the Sun, but its heat-retaining effect is stronger.

Like greenhouse gases, soot is a by-product of organic fuel combustion, and is belched out by automobiles, coal plants, agricultural burning, and the like. It only stays in the atmosphere for weeks (as opposed to greenhouse gases, which can remain in the atmosphere for hundreds of years), but constant emissions keep levels too high.

Global winds can carry the warming haze of atmospheric soot far from where it's produced. A study published this past March estimates that 75 percent of high-altitude atmospheric soot over the U.S. West Coast in spring comes from Asia. The same study estimated that in the Pacific Ocean region, the amount of springtime warming caused by soot is 40 percent of that attributed to CO₂ over the last century – a significant component. Since the Pacific Ocean drives much of the Earth's climate, this has a global impact, as well.

Most soot ends up in the atmosphere and its impact is greatest there, but it also does measurable damage on the ground when it finds its way to the Arctic. Snowflakes pick up soot particles as they fall, dirtying polar snow and sea ice and reducing reflectivity.

Even worse, there's an amplification effect. When polar snow and sea ice melt, warming is further accelerated because bare ground and ocean are less reflective. Mainly for this reason (and also for some lesser reasons), the Arctic is warming faster than the rest of the globe – a phenomenon called "polar amplification".

On the left, Sun's rays hit clean ice and are reflected back out to space. On the right, the Sun's rays hit sooty ice and are absorbed, melting the ice. Diagrams courtesy of NASA.

The 2003 NASA Science Brief "As Pure As Snow" provides a good overview of the problem. NASA also offers an accessible summary of a 2005 research paper on Arctic soot. You can see a Shuttle photograph of soot-blackened snow in the Russian City of Troitsk, courtesy of NASA. And just today, another study was published that confirms the high impact of soot on the Arctic.

So when you think about the dangers in burning fossil fuels for energy, you can add atmospheric soot to the list, right behind greenhouse gases.