I'm more than a little surprised by this post by Steven Goddard. His answer to my title question is yes. That he's wrong isn't very interesting. We all make mistakes, and particularly so when speaking outside areas that we've studied. The two main physical processes which show his error are interesting in their own right, and I'll take this chance to discuss them -- they are rivers (which say 200 years should be noticeable), and what happens to fresh water at 4 C (which says the memory is 1 year [oops, 6 months]).
First, I'll take a look at a less interesting error that minimal self-checking would have pointed to a difficulty. But that introduces a useful tool -- the 'sanity check'. Namely, he suggests that the reason Lake Superior is still cold is because it's so large that it is still adjusting to the end of the last ice age. That's about 10,000 years ago. Ok, suppose this line of reasoning is true. While Superior is large, is it tiny compared to the oceans. If Superior takes 10,000+ years to adjust, something 10 times bigger should take 100,000+ years to adjust. The ocean is about 100,000 times larger (in volume) than Lake Superior.
Showing posts with label ice age. Show all posts
Showing posts with label ice age. Show all posts
28 September 2010
11 April 2008
Sorts of ice age
In addition to there being types of ice, there are types of ice age. Again this seems odd, but again the differences become important if we try to understand the climate. Depending on which of the three types of ice age we're concerned with, the answer to the question 'Are we in an ice age now?' is either no, we've been in one for 2.5 million years, or we've been in one for about 35 million years.
The first type of ice age is probably the most familiar. It really is asking 'Is there a whole lot of ice in the northern hemisphere?'. A little more specifically, it is about how much ice mass there is. While the area of snow cover and sea ice cover can be large, we don't think of that as being an ice age for this type. The ice age for this is when most of Canada and the northern USA are under 1-2 miles (1-4 km) of ice. The last time we were in that sort of ice age was about 15,000 years ago, give or take. Certainly we were done with it by about 7,000 years ago. Call this ice age type 'ice burying Chicago'.
The next sort is the 'ice burying Greenland' ice age. We've been in that condition continually since about 2.5 million years ago. Greenland is larger than Texas or France, so counts for having some significant size. The question here is 'Is there substantial ice anywhere in the northern hemisphere?' Before about 2.5 million years ago, back to over about 250 million years ago, the answer was no.
Those two types of ice age are biased towards an interest in the northern hemisphere. Since I, and most of the world's people, live in the northern hemisphere, I'm not entirely opposed to the bias. But, since I also am concerned about ice and ice sheets in their own right, let's consider the more general question for ice age 'Is there substantial ice anywhere on the planet?'
The answer to this is yes, and has been so for about 35 million years. Antarctica, sitting down on and around the South Pole, has had substantial ice since about then.
Now it isn't the case that we always have lots of ice somewhere on the earth. Prior to that 35 million years ago, you'd have to go back more than another 200 million years to find an ice age by the third definition. Over the history of the earth, it appears that ice ages are an unusual condition for the earth. Most of the time (over 80% in fact), the earth doesn't have any significant ice. Obviously to see it this way, we have to be taking a very long view of the earth's history.
For thinking about current climate, then, we want to be careful about what sort of ice age we're talking about. Some people refer to us 'still coming out of the ice age'. They're wrong for all three sorts of ice age. We were done coming out of the last 'bury Chicago' sort of ice age 7,000 years ago. And we're still in the other two sorts. If they were to end, sea level would rise by about 7 meters (about 20 feet) for the 'ice in the northern hemisphere' ice age, or about 70 meters (over 200 feet) for 'ice in the world' ice age. Since some hundreds of millions of people live less than 20 feet above sea level, I'd rather not come out of even just the northern hemisphere sort of ice age any time soon.
The first type of ice age is probably the most familiar. It really is asking 'Is there a whole lot of ice in the northern hemisphere?'. A little more specifically, it is about how much ice mass there is. While the area of snow cover and sea ice cover can be large, we don't think of that as being an ice age for this type. The ice age for this is when most of Canada and the northern USA are under 1-2 miles (1-4 km) of ice. The last time we were in that sort of ice age was about 15,000 years ago, give or take. Certainly we were done with it by about 7,000 years ago. Call this ice age type 'ice burying Chicago'.
The next sort is the 'ice burying Greenland' ice age. We've been in that condition continually since about 2.5 million years ago. Greenland is larger than Texas or France, so counts for having some significant size. The question here is 'Is there substantial ice anywhere in the northern hemisphere?' Before about 2.5 million years ago, back to over about 250 million years ago, the answer was no.
Those two types of ice age are biased towards an interest in the northern hemisphere. Since I, and most of the world's people, live in the northern hemisphere, I'm not entirely opposed to the bias. But, since I also am concerned about ice and ice sheets in their own right, let's consider the more general question for ice age 'Is there substantial ice anywhere on the planet?'
The answer to this is yes, and has been so for about 35 million years. Antarctica, sitting down on and around the South Pole, has had substantial ice since about then.
Now it isn't the case that we always have lots of ice somewhere on the earth. Prior to that 35 million years ago, you'd have to go back more than another 200 million years to find an ice age by the third definition. Over the history of the earth, it appears that ice ages are an unusual condition for the earth. Most of the time (over 80% in fact), the earth doesn't have any significant ice. Obviously to see it this way, we have to be taking a very long view of the earth's history.
For thinking about current climate, then, we want to be careful about what sort of ice age we're talking about. Some people refer to us 'still coming out of the ice age'. They're wrong for all three sorts of ice age. We were done coming out of the last 'bury Chicago' sort of ice age 7,000 years ago. And we're still in the other two sorts. If they were to end, sea level would rise by about 7 meters (about 20 feet) for the 'ice in the northern hemisphere' ice age, or about 70 meters (over 200 feet) for 'ice in the world' ice age. Since some hundreds of millions of people live less than 20 feet above sea level, I'd rather not come out of even just the northern hemisphere sort of ice age any time soon.
04 April 2008
Types of ice
It sounds a bit odd to be talking about 'types' of ice. But media and net articles have included enough confusion that I've decided it's actually an important point.
Ice shows up in many parts of the climate system and is generally important when it does. So I'll describe the types of ice before getting in to the confusions. (Aside: the science that studies ice is glaciology).
In the atmosphere, ice crystals high in clouds can be important for starting rain formation. Of course ice falling from the sky, whether as snowflakes, ice pellets, or hail, can be important to know about as well.
On land, one of the less well-known ice types is permafrost -- ice that is in the soil and persists from year to year. This mostly happens in very cold areas such as northern Alaska, Canada, and Siberia. For more detail about permafrost, see the National Snow and Ice Data Center's (NSDIC) page http://nsidc.org/sotc/permafrost.html They also include a graphic of where you can find permafrost. In recent years, permafrost extent has been decreasing. Since ice is a somewhat hard material, this causes problems in areas where buildings and roads were built on top of permafrost.
The two better-known land ice types are glaciers and ice sheets. Glaciers are the ice that sits on mountains. The fact that they're on sloping surfaces is important for how they move. Again, the NSIDC has an introduction to glaciers, this one aimed better towards students than the permafrost. Ice sheets are the enormous masses of ice on land. They're so big that they swamp mountains and mountain ranges. Now there are only two ice sheets -- the Greenland and the Antarctic. Sometimes we talk of the East and West Antarctic ice sheets because a mountain range (the Transantarctic mountains) runs through the Antarctic ice sheet and does fairly well separate the flow of ice. During the last ice age, there was a large ice sheet across most of northern North America, called the Laurentide ice sheet. Both ice sheets are losing mass, as are most glaciers. One reason for concern about this is that as the ice melts, its meltwater flows into the ocean and raises sea level.
On water we have three different types of ice to think about. Perhaps the oddest is ice shelves. This is ice that is floating on water, but which is attached to ice on land. The land part feeds the floating part. Several Antarctic ice shelves have collapsed into the ocean in the last few years. Icebergs are what we get when a chunk of an ice shelf breaks off. They can be as large as cities and small states or countries. The large ones, though, are in the Antarctic only. The Greenland icebergs are much smaller, though more than enough to sink the Titanic. The International Iceberg Patrol monitors the locations of these North Atlantic icebergs.
The last happens when the water freezes -- sea ice. Sea ice floats and gets moved by winds and water currents. It is important for predicting the weather, so you can see today's coverage at the NOAA/NWS Marine Modeling and Analysis Branch web page. Last fall, the Arctic experienced a record minimum in its extent at the end of the summer. The NSIDC has collected information about the record at http://nsidc.org/news/press/2007_seaiceminimum/20070810_index.html On the other end of the earth, the Antarctic ice pack has been showing a small increase in its extent, see the Crysophere Today for information about the anomaly (how much more or less ice there is compared to what's normal for the time of year) in ice coverage for both the Arctic and Antarctic. (The Antarctic is at the bottom of the page.)
So now for the confusions some have had. When our concern is about sea level, the type of ice we care about is land ice -- the ice sheets and glaciers mostly. When these types melt, sea level rises. If all of the Greenland ice sheet melted, sea level would rise about 6 meters (about 20 feet), enough to put much of Florida, for instance, below sea level. Glaciologists have observed that both Greenland and Antarctica ice sheets are melting. A research question is just how fast they will melt in the future. Most glaciers are also melting.
That's straightforward. What some have confused is 'Antarctic ice', where they're thinking about sea level changes (so should be concerned about land ice) and then looking at the sea ice instead. Sea ice has little to do with sea level. In terms of total sea ice trends, you can look again at Crysophere Today and compare how large the Antarctic positive anomaly is to the Arctic negative anomaly.
A different confusion was someone who compared the February ice cover in the Arctic (near the time of its maximum extent -- it's cold in the Arctic in the winter!) to the cover last fall (near the time of minimum coverage -- after the summer's heat had time to melt as much ice as it could). When we're looking for changes for climate, we certainly have to pay attention to what time of year it is!
If anyone says there's more (or less) ice, be sure that they know which type of ice they're talking about. Also check that they're comparing the same time of year.
Ice shows up in many parts of the climate system and is generally important when it does. So I'll describe the types of ice before getting in to the confusions. (Aside: the science that studies ice is glaciology).
In the atmosphere, ice crystals high in clouds can be important for starting rain formation. Of course ice falling from the sky, whether as snowflakes, ice pellets, or hail, can be important to know about as well.
On land, one of the less well-known ice types is permafrost -- ice that is in the soil and persists from year to year. This mostly happens in very cold areas such as northern Alaska, Canada, and Siberia. For more detail about permafrost, see the National Snow and Ice Data Center's (NSDIC) page http://nsidc.org/sotc/permafrost.html They also include a graphic of where you can find permafrost. In recent years, permafrost extent has been decreasing. Since ice is a somewhat hard material, this causes problems in areas where buildings and roads were built on top of permafrost.
The two better-known land ice types are glaciers and ice sheets. Glaciers are the ice that sits on mountains. The fact that they're on sloping surfaces is important for how they move. Again, the NSIDC has an introduction to glaciers, this one aimed better towards students than the permafrost. Ice sheets are the enormous masses of ice on land. They're so big that they swamp mountains and mountain ranges. Now there are only two ice sheets -- the Greenland and the Antarctic. Sometimes we talk of the East and West Antarctic ice sheets because a mountain range (the Transantarctic mountains) runs through the Antarctic ice sheet and does fairly well separate the flow of ice. During the last ice age, there was a large ice sheet across most of northern North America, called the Laurentide ice sheet. Both ice sheets are losing mass, as are most glaciers. One reason for concern about this is that as the ice melts, its meltwater flows into the ocean and raises sea level.
On water we have three different types of ice to think about. Perhaps the oddest is ice shelves. This is ice that is floating on water, but which is attached to ice on land. The land part feeds the floating part. Several Antarctic ice shelves have collapsed into the ocean in the last few years. Icebergs are what we get when a chunk of an ice shelf breaks off. They can be as large as cities and small states or countries. The large ones, though, are in the Antarctic only. The Greenland icebergs are much smaller, though more than enough to sink the Titanic. The International Iceberg Patrol monitors the locations of these North Atlantic icebergs.
The last happens when the water freezes -- sea ice. Sea ice floats and gets moved by winds and water currents. It is important for predicting the weather, so you can see today's coverage at the NOAA/NWS Marine Modeling and Analysis Branch web page. Last fall, the Arctic experienced a record minimum in its extent at the end of the summer. The NSIDC has collected information about the record at http://nsidc.org/news/press/2007_seaiceminimum/20070810_index.html On the other end of the earth, the Antarctic ice pack has been showing a small increase in its extent, see the Crysophere Today for information about the anomaly (how much more or less ice there is compared to what's normal for the time of year) in ice coverage for both the Arctic and Antarctic. (The Antarctic is at the bottom of the page.)
So now for the confusions some have had. When our concern is about sea level, the type of ice we care about is land ice -- the ice sheets and glaciers mostly. When these types melt, sea level rises. If all of the Greenland ice sheet melted, sea level would rise about 6 meters (about 20 feet), enough to put much of Florida, for instance, below sea level. Glaciologists have observed that both Greenland and Antarctica ice sheets are melting. A research question is just how fast they will melt in the future. Most glaciers are also melting.
That's straightforward. What some have confused is 'Antarctic ice', where they're thinking about sea level changes (so should be concerned about land ice) and then looking at the sea ice instead. Sea ice has little to do with sea level. In terms of total sea ice trends, you can look again at Crysophere Today and compare how large the Antarctic positive anomaly is to the Arctic negative anomaly.
A different confusion was someone who compared the February ice cover in the Arctic (near the time of its maximum extent -- it's cold in the Arctic in the winter!) to the cover last fall (near the time of minimum coverage -- after the summer's heat had time to melt as much ice as it could). When we're looking for changes for climate, we certainly have to pay attention to what time of year it is!
If anyone says there's more (or less) ice, be sure that they know which type of ice they're talking about. Also check that they're comparing the same time of year.
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