Service Interruption

07 July 2006

Does Al Gore get the science right in the movie An Inconvenient Truth?

We know that a lot of people wonder if the science presented in An Inconvenient Truth is correct. NSIDC scientists Walt Meier and Ted Scambos answered some Frequently Asked Questions about the snow and ice science presented in the movie.

As a scientist who studies the climate, what do you think of the movie?

TED: I think An Inconvenient Truth does an excellent job of outlining the science behind global warming and the challenges society faces in the coming century because of it. 

WALT: I agree. I think Gore has the basic message right. But we thought we could clarify a few things about the information concerning snow, ice, and the poles.

The movie shows glaciers calving into the water as Gore discusses the effects of warming on the glaciers. But don't glaciers always calve into the water when they end there?

WALT: Yes, glaciers and ice streams that terminate over water are called tidewater glaciers. They are always calving ice into water; these chunks are one source of icebergs. If things are in equilibrium, there is no loss of ice because what is calved off is replaced by snowfall further up the glacier. However, in recent years, almost all glaciers are in retreat and losing mass. Some glaciers have retreated so far that they no longer reach the water and are no longer considered tidewater glaciers.

See an example of a glacier photograph pair.

TED: Just to clarify, Mr. Gore's characterization of recent dynamic events in Arctic glaciers does rest firmly on observational evidence from a variety of sources, not just on glacier pair photos like those mentioned above. It’s true that every glacierized region on Earth is experiencing retreat and thinning.

The movie discusses decreasing sea ice in the Arctic and how it will affect climate and wildlife. Is it really that important?

WALT: Arctic sea ice is something NSIDC watches carefully, especially because of its effect on global climate. Sea ice helps regulate climate because it acts like a mirror, deflecting incoming solar energy and helping to balance the Earth’s temperatures. As the sea ice disappears, dark ocean water is exposed to the sun’s energy, and the Arctic’s ability to cool our planet also disappears.

To learn more about the recent, record-breaking disappearance of sea ice, see NSIDC’s Sea Ice Decline Intensifies press release. To learn more about sea ice, its role in climate, and the animals and people that depend on it for survival, see All About Sea Ice.

Why doesn't the movie mention anything about the sea ice in Antarctica?

WALT: The Antarctic sea ice is not showing the dramatic downward trend that we are seeing in the Arctic. There are a couple reasons for this. First, the circulation of the atmosphere and oceans isolates the ice-covered ocean around Antarctica, keeping things colder. So, global warming effects have not yet been pronounced there. And, unlike the sea ice in the Arctic, much of which stays around the entire year, most of the sea ice in the Antarctic already melts away during the summer so any summer warming would not have as much of an impact. 

However, sea ice around Antarctica plays a key role in ocean circulation by producing dense cold water that sinks to the ocean floor and helps drive the ocean’s "conveyor belt" circulation. Sea ice is also key to biology in the region, which is one of the most productive marine environments in the world. There already have been some changes noted in the timing of when the sea ice forms and melts around the Antarctic Peninsula, which has shown significant warming. This warming contributed to the collapse of the Larson B Ice Shelf in 2002.

To view the animation of the Larson B collapse and to read more about the breakup, see Larsen Ice Shelf Breakup Events.

TED: NSIDC studied the collapse of the Larson B Ice Shelf extensively.  As Gore mentions in the movie, ice shelves can act almost like a cork in a bottle, holding back the ice behind them. We recently set up a new study to learn how ice shelves melt and collapse. To learn more about the expedition to Antarctica, see the IceTrek Web pages.

The movie talks about the shrinking Greenland and Antarctic ice sheets, but I heard somewhere that scientists have found that the ice is actually gaining mass. Which is correct?

WALT: It is true that both Greenland and Antarctica have gained mass, but only at the high elevations in their interior. This is because of increased snowfall, which even though it may seem counterintuitive, is actually expected under warmer conditions.  However, both have been losing ice at the coast at increasing rates in recent years. In Greenland, it is becoming apparent that there is a net loss of ice. In Antarctica, the data are inconclusive, although the most recent results point to a loss. Under continued warming conditions, a net loss of ice is assured and rising sea levels would follow.

TED: Walt is right. We know from looking at past records of climate changes during the ice ages that eventually warming leads to drastic losses of ice from both the Greenland and Antarctic ice sheets, as well as several other ice sheets that don't exist, anymore. Konrad Steffen, a scientist here at the University of Colorado at Boulder, does a lot of work on Greenland’s ice sheet. For more information, see Greenland Melt Extent, 2005.

The movie shows coastal areas flooded because of the melting polar and Greenland ice caps. When will this happen?

WALT: Coastal flooding is not something that will happen right away. Initially, scientists estimated that it would take at least 1,000 years for the Greenland and West Antarctic ice sheets to melt, with the East Antarctic ice sheet taking much longer. However, the melt has increased in recent years much more quickly than scientists initially expected. Now, a rough estimate might be 500 years for Greenland and West Antarctica to melt completely; it's much less likely that East Antarctica would melt. However, well before they melt completely, the ongoing melting of these massive ice sheets will slowly increase sea level. That, in turn, will lead to damaging tides and storm surges.  

It’s also important to note that even though the full impact of that gradual melting won’t be for 500 years or so, we are reaching a point where we can’t turn back. The system is slow to change, but the change is somewhat unstoppable once it gets going. Unless we quickly reduce the present rate of carbon dioxide increase and subsequent temperature rise, we will be committing ourselves and our planet to that melting, and to the rise in sea level that will follow.

When Gore rides the lift to the top of the carbon dioxide curve and suggests that the temperature might keep rising right along with it, is that correct?

TED: Records taken from ice cores do show the close relationship between carbon dioxide and temperature over the past 650,000 years. Gore basically says that the full relationship is very complicated, but that the main point is carbon dioxide and temperature have always moved together. This implies that, in the past, when carbon dioxide has increased it has led directly to a warmer Earth.

However, past changes in carbon dioxide levels are at least initially an effect of abrupt climate change, not a cause. What happens in an ice age is a sudden shift in ocean circulation, pushed by a cooling trend in the northern hemisphere because of periodic shifts in the Earth's tilt and orbit. The trigger that causes the ice age is in the Northern Hemisphere, and the huge Northern forests and ecosystems begin to slow down as tundra spreads across Asia and North America. But ice cores from Greenland and Antarctica tell us that both hemispheres go through ice ages and interglacial warm periods together.

You might wonder, why would Antarctica go into an ice age at all? The orbital and tilt-related changes that initiate ice ages have opposite effects on the two hemispheres. Trends that push the North to colder climes push the South towards warmer conditions. The opposite happens in a warming period: the North leads the way. However, the carbon dioxide eventually drags the South towards warm conditions, as well.

The globalizing agent responsible for bringing both hemispheres through the ice-age cycles together is carbon dioxide. This is the best observational proof that the changes we are seeing now are similar to past global effects of carbon dioxide on the climate. In fact, somewhat smaller changes than what we are seeing now can have a large effect on climate. As the northern ecosystems slow down, carbon dioxide in our atmosphere drops, causing further cooling—globally—despite the tendency towards warming in the other hemisphere.

Where can I read more scientific reaction to the movie, especially about aspects of the science that you don’t cover here?

WALT:, a non-profit, non-governmental site run by scientists, has a good entry on the movie.  See

I still want more information. Who can I contact?

Contact NSIDC User Services at +1 303 492.6199 or