Global warming might leave some places colder
Thorsten Markus talks about Antarctic sea ice and global warming.
Thorsten Markus, of NASA’s Goddard Space Flight Center, uses satellite data to calculate precipitation rate for the far southern part of the globe. Based on this research, he believes that Antarctic sea ice might increase, as Earth’s atmosphere continues to warm in the coming century. Earth & Sky’s Abby Frank spoke to him in 2005.
Frank: How is warmer air increasing the volume of sea ice in the Antarctic’s Southern Ocean? It seems very counterintuitive.
Markus: Yes, it does indeed. The main reason, and this is what our study has shown, is that with warmer air temperatures, the air is able to hold more moisture, which results in increased precipitation. The precipitation adds a layer of snow on top of the sea ice. With this snow layer on top of the sea ice, part of the sea ice is submerged below the sea level, and then the snow becomes ice and it makes thicker ice.
Frank: Why won’t we see the same effect in the Arctic?
Markus: This increase in precipitation with warmer air temperatures is not so clearly seen in the Arctic as it is in the Antarctic. We have different weather systems in the Arctic and in the Antarctic. But more important, probably, is that the ice in the Antarctic is thinner than in the Arctic. In the Antarctic, the average ice thickness is about half a meter to a meter. In the Arctic, the average ice thickness is closer to three meters. This is the result of the ocean underneath the sea ice.The deep water of the Antarctic is relatively warm. Because the sea ice is thinner in the Antarctic to begin with, an increased snow load has a bigger impact on the sea ice than it has in the Arctic.
Frank: So, the sea water is warmer in the Antarctic than in the Arctic?
Markus: It’s an oceanographic feature that we have a cold surface layer, which is close to freezing, in the Arctic as well as in the Antarctic. The difference is that, in the Antarctic, we have what is called warm deep water underneath this cold, surface layer, which constantly provides heat to the surface layer. This reduces the freezing rate of the sea ice. So, we have a thinner ice cover in the Antarctic than we have in the Arctic. In a nutshell, without this warm deep water in the Southern Ocean, we would have ice thickness similar to the ice thickness in the Arctic.
Frank: Can we expect this trend to continue as the climate gets even warmer, with ice melting at the North Pole and building up at the South Pole?
Markus: This is what studies suggest, if temperatures are increasing and precipitation with it is increasing as well. Very recent studies by other scientists have shown this correlation between warmer air temperatures and precipitation in the southern hemisphere. So, if this trend exists, we will likely see a further increase in the sea ice cover in the Antarctic.
Frank: How accurate is your model?
Markus: That’s a very good question. I wish I had a great answer for that. How do you determine accuracy in terms of trends? The problem is, only the future will tell us whether we were right or not. We can reproduce the current state of the sea ice pretty accurately. So, we have some confidence in the results. Nevertheless, there are still lots of parameterizations that we cannot model sufficiently yet. So, there are some simplifications in the model.
Frank: How was satellite data used in your research?
Markus: The problem is that there are basically no precipitation measurements in the Antarctic. In the United States, it’s easy. Every city has some measurement system. In the Antarctic, there are hardly any ground measurements. So estimates of precipitation relied primarily on model calculations. And the models are not really that great in the Southern Ocean — partially because of the lack of validation data sets. For this reason, we have developed an algorithm to measure the snow depth on sea ice from satellite passive microwave data. And this snow depth is a proxy to estimate polar precipitation. These data sets are assimilated into the precipitation model so that we have a better-modeled precipitation rate than without this data set.
Frank: What impact will this increase in Antarctic sea ice have on other areas of the globe?
Markus: There are many processes going on in the whole climate system. One is the ice albedo feedback. If you have more ice in the Antarctic, it could cause a cooling effect, because ice is white and it reflects more sunlight, and this may balance some of the decrease in sea ice in the Arctic.
But I think probably more importantly is the effect of the ocean circulation. Our climate relies heavily on our so-called thermohaline circulation”:http://en.wikipedia.org/wiki/Thermohaline_circulation, the overall, general ocean circulation. In the future, it may not be the way it is right now. It hasn’t been this way in the past. And this can cause a dramatic change in our climate system, because our ocean is a buffer. We have weather changes from day to day, from week to week, from year to year, but we have our big buffer, which is the ocean, which helps the climate to stay relatively stable. But if the ocean circulation changes, then even more dramatic changes in the climate system could happen.
Frank: By dramatic changes, do you mean dramatically cooler temperatures in some places?
Markus: Potentially, yes. The misconception is that global warming means warmer air temperatures everywhere. This is certainly not the case. This is why some people think, “Oh yeah, global increase in air temperature of one degree is really nothing. If we have 85 and in the future we have 86, it doesn’t make a big difference.” And it does not, but the temperature changes are different regionally.
Frank: Is that the significance of your findings, that global warming will have drastically different effects in different places?
Markus: I think that’s probably the main conclusion that could be drawn here, because it just shows that global warming is not so simple as people like to assume. This reminds me of the Michael Crichton book, State of Fear, because he questions the whole global warming aspect by asking how it can be that in one area temperatures are increasing and in a different area temperatures are decreasing? There are many processes involved. It’s not just, “We have warm air temperatures so everything should become warmer.”
