Ice is a key indicator of global warming and climate change for the rather obvious reason that it melts. And since it is so important this will be the introductory article in a series of Climate Truth pieces discussing the current state of the science related to ice, and its importance to the feedback mechanisms that will dictate our future climate change.
To begin with, it’s important to understand that when we talk about ice, we’re actually talking about a lot of different things. The “cryosphere” includes snow, river and lake ice, sea ice, glaciers and ice caps, ice shelves, ice sheets, and frozen ground. The form, and the location, of the ice is critical to the interpretation of any observed trends. For example, snow falling in temperate zones tends to melt relatively quickly, but snow falling on polar regions enters a whole new dynamic that affects the amount and quality of ice. You can see the different types in the graphic here, with the relative time frames they are important in the bars at the bottom.
So when it comes to ice, there are several ways to characterize it.
First, some ice is on land and some ice is floating on the water. When scientists talk about icebergs and sea ice they obviously are talking about ice floating on the ocean (and lakes and rivers). When sea ice melts it may affect temperatures and salinity but it doesn’t change water levels due to the phenomenon of displacement (in short, your iced tea doesn’t overflow the glass when the ice cubes melt). Land ice, on the other hand, sits on top of continents and islands, so when it melts it runs off and can significantly raise sea levels over time.
The major locations of sea ice are the Arctic sea and the ocean surrounding Antarctica. Sea ice tends to grow in the winter because the temperature is really cold, but then melts back in summer as the temperatures warm up. Major areas of land ice occur on Greenland, the land masses within the Arctic Circle (e.g., northern Canada, Russia, Alaska, Scandinavia), and, of course, Antarctica. Substantial amounts of land ice are also found in the world’s mountain glaciers. Overall, nearly 70% of the world’s freshwater is frozen in ice sheets, glaciers, permanent snow cover, and permafrost. That’s a lot of ice. And guess what, it’s melting.
But wait, there’s more. More potential for confusion, that is. In addition to the above there are even more ways to characterize ice.
When talking about ice sheets or sea ice, for instance, you may hear terms such as ice extent, area, mass, volume, and age. Each is a measure of different characteristics that need to be taken into account, and each is appropriate in different situations depending on what question is being asked. For example, sea ice extent (usually defined as area of ocean covered by at least 15% ice) may be about the same from year to year but the actual mass or volume could be much less in those years. Think of putting a thin sheet on a bed versus putting a thick down comforter on the same size bed…the extent (or area) may be the same, but clearly the thicker one is going to provide more warmth. In short, thickness matters. Thickness is also related to age of ice. Looking at the amount of “multi-year” ice, usually defined as ice that stays frozen from year to year rather than melting in the summer months, older ice is usually a better indicator of the health of an ice sheet than simply the areal extent. Obviously, a thin, young ice sheet will be quicker to disappear than a thicker, old ice sheet.
One more point before wrapping up this introduction to ice. It’s important to remember that the Arctic is water surrounded by land while the Antarctic is land surrounded by water. This makes a huge difference in the dynamics of these two regions, especially when you consider that the northern hemisphere contains a large amount of land mass and most of the earth’s population centers (and CO2 emitters) while the southern hemisphere contains a larger amount of oceanic area and significantly less inhabitants (and emissions). And even in Antarctica there is a big difference between the dynamics of West Antarctica and East Antarctica.
But that is a topic for a future post. For now I’ll leave you with a snapshot of the state of the ice on the planet (see here). All of these indicators below would be expected to decrease as the planet gets warmer. And they are doing exactly that. I’ll explain why and what it all means as the series of Climate Truth posts on ice continues.
© David K, June 2011
This post is part of a series in The Truth About Global Warming, located at climatetruth.gather.com, which is dedicated to explaining what we know, and what we don’t know, about climate change.
I also have set up a separate group called “Exposing Climate Denialism – A Guide to Tactics and Tall Tales,” located at climatelies.gather.com for those who want to read about some of the intentional disinformation used by climate denialists to confuse the public about the state of climate science.