Understanding the Poles of Earth, the Moon, and Mars
National Museum of Science and Industry, London, England
Al Gore, on the inside front cover of his book Inconvenient Truth displays a familiar image (Figure 6.1). He points out that it is the most published image of anything, Earth in particular, that you will come across. It’s a photograph that was taken on December 7, 1972, by the astronauts on the last Apollo mission, Apollo 17, soon after the astronauts had left Earth orbit. It’s an impressive photograph because it’s one of the very few where the Sun is behind the camera so you see a fully illuminated globe. It’s also impressive from the point of
view of those of us interested in the poles, particularly the Antarctic, because as you look carefully you’ll see that the picture was taken well out of the equatorial plane of the planet and you can see Antarctica quite prominently at the bottom of the globe.
Quite honestly, you could devote the whole of this paper to simply discussing the many facets of this image. It should be pointed out that in spite of the best efforts to discover life on other planets, we have not actually done that yet. In order to understand this object, you not only need geologists and physicists and chemists, but you need biologists. The sort of green color you can see on Africa is, of course, due to biology. To fully understand this image you also need economists, technologists, sociologists, and what have you. You need to assemble all of these scientists together to understand this object because it operates blissfully unaware that we’ve divided it into little pieces and studied them separately. We need to recognize that its various components, the atmosphere, the ocean, the ice, the biology, the humans, all interact in hugely complex non-linear ways.
It can be argued that Earth is, as far as we know, the most complex object in the universe and therefore a really worthy object of our study, not least because it happens to be our home. It’s a puzzle to me that more Earth images were not taken on the Apollo missions. Figure 6.2 is one of the relatively few photographs of Earth that were taken by the Apollo astronauts. This lack of Earth images is a bit puzzling until one accepts that the whole point of the Apollo program was to leave Earth and to get to the Moon, which was therefore the focus of everybody’s attention.
The red dot shows roughly where Apollo 17 was heading. It got there a few days after that photograph was taken. Then on December 14, 1972, we reached the end of the heroic age when Gene Cernan and Jack Schmitt lifted off from the Moon. Just in the same way that the exploration of Antarctica had its heroic age, this remains the end of the heroic first age of lunar exploration.
What is particularly interesting is that the first human landing on the Moon took place just under 12 years after the launch of the Soviet Union’s Sputnik 1, which was launched on the fourth of October 1957, heralding the start of the space age that we all celebrate today.
THE FIRST INTERNATIONAL POLAR YEAR
Sputnik 1 was part of the International Geophysical Year (IGY; 1957 to 1958), which had it’s origins in something that had happened considerably earlier, in the 1880s. The first International Polar Year (1882 to 1883) was proposed by George Neuymeyer but was actually developed, although not completely executed because he did not live to see it completely, by an Austrian naval lieutenant, Karl Weyprecht. He developed the principles of the International Polar Year as follows.
Nations should collaborate,
Coordinated research expeditions using standardized instruments and methods would give a bigger bang for their collective buck,
Observations should be over at least one annual cycle, and
Observations should be a synchronized.
This showed great foresight for its time, but it has remained the basis of all the international collaboration on terrestrial, and if you like, space research that has followed.
There were 12 nations involved, undertaking a total of 12 expeditions to the Arctic and 3 to the Southern Ocean and the Antarctic. Fourteen meteorological stations were operated and there was a wide range of science undertaken: polar meteorology, atmospheric electricity, geomagnetism, auroral studies, ocean currents, tides, ice motion, and so on. All benefited from this joint collective standardized approach.
The Second International Polar Year
Fifty years later there was a second International Polar Year, organized by the World Meteorological Organization, which had also been involved in the earlier event. Forty nations participated with 40 Arctic observ-
ing sites. The United States established the first inland Arctic research stations, so beginning the penetration of the Antarctic for research purposes. Meteorology, including the “jet stream” and ionospheric studies were the subjects of a lot of the effort. The initiative was not quite as successful as it might have been because it took place during the Great Depression, and there simply was not the financing available for it to be anymore than it was.
The International Geophysical Year
Twenty-five years later, after World War II, and with the huge surge of technological and scientific advance that that stimulated, it was decided that it would be a good idea to have a third International Polar Year. This grew to become the International Geophysical Year. It was organized by the World Meteorological Organization and by the International Council of Science, or Scientific Unions (ICSU) as it was in those days, and involved 67 nations and 8,000 stations; 12 nations went to the Antarctic and set up 40 stations, mainly around the coastline. Eighty thousand scientists and volunteers were involved and a very broad range of science was addressed.
It took place, of course, in the shadow of the Cold War, but it fostered that wonderfully creative mix of both international cooperation and rivalry that stimulated huge amounts of progress. Some examples of what emerged from the IGY are:
The discovery of the Van Allen belts,
The first measurements of the thickness of the Antarctic ice sheet—thickness in nature, if you like, of the Antarctic ice sheet, and
Establishment of the first Arctic and Antarctic permanent bases and research programs, which have endured since.
Furthermore, the establishment of:
The scientific committees for Antarctic research, SCAR, and for ocean research, SCOR,
World data centers,
World Climate research programs, and
The International Geosphere-Biosphere program
all have their origins in the IGY.
Basically, all of the international coordinated Earth-system science that you see today owes its origins to this historical thread. Of course, another extremely important outcome of the IGY was the Antarctic Treaty system. The nations with land claims in the Antarctic, particularly those with disputed land claims, agreed to set them aside during the period of the IGY. They then decided that “if we can do it for a couple of years why don’t we do it in perpetuity,” and indeed that is the way that the government of the Antarctic was established. This joint government now involves 34 nations and has been an extraordinary successful, if slightly arcane, way of managing a major part of the planet, which is reserved for peace and science.
IGY made the news. The public was aware of it. It was a very exciting time, not least because of the beginning of space age and the birth of space research. However, it was also a time when the public took a greater interest, at least for a short while, in science and what science can do.
The beginning of what many people recognize as quantitative Earth observation started with SEASAT in 1978. This was followed by a whole series of other satellites which have provided us with a unique view of Earth. To use a term coined by Hans-Joachim Schellnhuber at the Potsdam Institute for Climate Impact Research, we can now view Earth through a “macroscope” (Figure 6.5). Just as you use microscope to study something that is very small compared with you, you can use a macroscope to study something that is very large compared with you. Earth is that object.
With the convergence of the ground tracks at polar orbiting satellites at the poles, not only do you get a
fantastic view of what is going on over the whole Earth if you build the right instruments and put them on the right satellites, but you also get a spectacularly good view of what is going on in the polar regions.
What have we learned from this? It is a combination of the work done by all of the scientists involved in the space-Earth observation initiatives over the past three decades and also all of the work that has gone on the ground and in the atmosphere using ships, aircraft and research stations. What we have learned is that human-induced climate change is real and serious.
If you’re looking for a dramatic non-linear effect of warming, then watching a piece of ice melt is pretty dramatic, and in a warming world, ice retreats. It does not necessarily have to melt, at least not initially, but it can slide off into the ocean and do things and then melt at its leisure; so ice retreats. Now one of the things that you often hear is that the polar regions see an amplification of global warming, and this is true. The fastest warming spots on the planet are in Alaska, Siberia, and the Antarctic Peninsula. At least in part, this is because as you melt white ice or snow, it reveals dark ocean or land, so instead of reflecting away 80 percent of the incoming solar heat or light, you absorb it.
One thing to bear in mind is that for half the year in the polar regions it is dark, and if the ice cover has been removed the ocean and the atmosphere are far better coupled than they would otherwise be. Quite a lot of that heat comes back out again. However, it is not entirely obvious which way this will work out. It is the difference of two large numbers, but it does turn out that by and large the ice-albedo feedback does have its affect, and this is why we have seen this extra warming in the polar regions. However, because you get this amplification in the polar regions, the system amplifies the noise as well as any systematic signal. It’s not at all obvious that you would detect climate change or human-induced climate change more easily in the polar regions than in the tropics. The signal to noise ratio issue is not fully resolved. However, if there are impacts of humans on the planet, they will have a big impact in the polar regions.
Figure 6.6 shows the sequence of satellite-derived minimum-Arctic-sea-ice measurements that extends from the summer minimum of 1979 through to the summer minimum of 2005. Despite the large amount of inter-annual variability, it can be seen that there has been a steady decline of about 25 percent over that period. In 2005 there was bit of a dip, and people speculated about whether they’re seeing an acceleration of the loss of sea ice, especially because of a rather dramatic decline in the multi-year ice. The thicker ice that survived several annual cycles was noted, but in 2006, measurements were back up on the curve again, and then last year (2007) there was an extraordinary decrease.
The previous minimum of 5 million square kilometers is shown on the right in Figure 6.7. The 2007 minimum of 4 million square kilometers is shown on the left. This is an excellent example of what satellite data can tell us.
The image in Figure 6.8 was taken by passive microwave instrument. You can see the pole with a sort of black hole where there is no observation and you can clearly see Greenland. You see the ice melting back to the minimum, and if you know where to look, the northwest and northeast passages open up in a way that is quite unprecedented. Somebody decided to see if they could sail a sailing boat through the Northwest Passage last year, and they found themselves in the
Pacific without having seen a single piece of ice. That is a measure of the dramatic change that has been seen. The Northwest Passage is marked in yellow on the left. The Northeast Passage, which still required a little bit of ice breaking to get through last year, is marked in blue on the right. Trying to get ships through these passages was something that the British Navy and others struggled to do for the best part of 50 years. This is a good thing for those who wish to transport goods during these brief summer months, and in South Korea
the shipyards are building vessels for this purpose. On the other hand, it’s not such a good thing if you’re a polar bear or if you’re a frontier person whose livelihood depends on having sea ice to fish from or to catch seal from.
Floating sea ice, if it melts, does not change sea level. Archimedes would have understood that. Up on the Greenland ice sheet, the area of summer melting has increased in size and progressed steadily northwards. The summer melt area has increase dramatically over the time period during which the area has been monitored by satellites. This surface melting is not just a few little puddles that get your feet wet. These are torrents of water that can make their way down through the ice sheet, lubricate the underside of it and indeed cause it to accelerate in its gravitational extrusion towards the periphery of the continent and the ocean. There has been some increased snowfall in the interior in Greenland, but not enough to compensate for this increased acceleration. The ice discharged by melting and sliding has increased dramatically between 1996 and 2005. It is making a significant contribution now to sea level rise, which previously was dominated by thermal expansion and the melting of glaciers.
Synthetic Aperture Radar has proved to be one of the most extraordinary of instruments available to the remote sensing community and has absolutely transformed our ability to monitor the movement of ice. However, a new weapon has arrived in our armory recently and that is the extraordinary capacity to measure the gravity field, and indeed the changes—very, very subtle changes—in gravity due to the changing distribution of the ice mass, or indeed the loss of ice in this particular case.
The graph shown in Figure 6.9, derived from gravity data obtained by the Gravity Recovery and Climate Experiment (GRACE) mission (twin satellites launched in March 2002 that are making detailed measurements of Earth’s gravity fields) show the rate of change of mass of the Greenland ice sheet—quite an extraordinary feat.
Let us look at the Antarctic now. One of the things that has happened in the past 15 years is that the “Westerlies” (prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing towards the poles) have increased in intensity around the Antarctic. It has also been noted that the place that has warmed the fastest around the planet (2½ degrees or more) in the last 40 years is the Antarctic Peninsula. Indeed, there has been a study that indicates that these things are connected. With the intensification of the Westerlies, there have been a greater number of events where warm winds make it over the Antarctic Peninsula, and a succession of ice shelf collapses all due to human-induced global warming, combined with an effect of the ozone hole.
Whether or not you accept that, there is no doubt that there has been warming, and that 90 percent of the glaciers on the peninsula are in retreat. The ecology is also changing. For example, penguin colonies and distributions are shifting in response, indicating that there is a major upheaval going on. However, one thing that can be done when an ice shelf has collapsed is take a research ship into where it was, get some samples of the sediment from underneath it, and try and figure out whether it had collapsed in the past. When that has been done, it has been found that the northern
most ice shelves had collapsed perfectly naturally in a climatic fluctuation 3,000 to 5,000 years ago. However, when it comes to the Larsen ice shelf which collapsed in 2002, allowing sediment to be removed from that area of the continental shelf, it was found that that ice shelf had been in place for at least 10,000 years and probably longer. Warming is reaching locations it has not reached before.
One of the things that the Larsen ice shelf collapse did for us was resolve a long-standing dispute amongst glaciologists as to whether these ice shelves provide a back pressure obstructing the flow of the feed glaciers. Monitoring the behaviors of those feed glaciers after the Larsen B collapse indicated that this was in fact the case. Just a month ago on the western side of the peninsula the Wilkins ice shelf suddenly lost a very substantial segment of ice. Almost exactly a month ago, the ice was still in place. One day later, an almost explosive collapse of the ice took place, and it is now drifting off to sea. The thing about the Wilkins ice shelf is that it is 5 degrees of latitude further south than Larsen B. This suggests the warming is penetrating yet further south.
Figure 6.10 is a high resolution image of these huge blocks of ice moving out to sea. Interestingly Larsen B
had been weakened by melt ponds on its surface that had broken up its structure and fabric.
If you look at these pictures of the Wilkins ice shelf that were taken by a research aircraft just a week or so ago (Figure 6.11), you’ll see that it seems to have suffered a different sort of fracture. It will be very interesting to see what the glaciologists make of this. They are flying along a fracture between two major pieces, but look at these extraordinary perfect pieces of ice shelf that have broken almost at right angles and with almost plain “R” cracks.
In Figure 6.12, the area circled in red is part of the West Antarctic ice sheet, the so-called Amundsen Sea Embayment. Radar altimetry has show us for some while that the drainage basins marked H and G are discharging ice very dramatically. The green colors show a substantial loss of ice. Notice that over in east Antarctica there are a couple of other areas that are discharging ice as well.
It turns out that those areas have a very substantial volume of ice sitting on rock below sea level. The West Antarctic is a marine ice sheet (Figure 6.13); it is ice that is sitting on rock up to 2 kilometers below sea level, which means this is a substantial hydrostatic uplift, trying to lift it off the rock. It is very heavy, so that uplift is not winning at present. As that ice begins to slide, it is apparent that the very-high-pressure water
at the ground line can begin to force its way in, under the ice sheet.
What we do not know, because we do not have the physics to put into the models, having never seen a marine ice sheet collapse before, is whether this retreat will come to a halt, or whether it will continue until all of the ice that could discharge has discharged. Indeed, if that were to happen, how long would that take?
Using Interferometric Synthetic Aperture Radar, you can see where major discharges are taking place around the Antarctic. It has been said that the increased snowfall on the Antarctic domes, the high parts of the ice sheet, would compensate for some of the losses. Indeed that has been assumed by the Intergovernmental Panel on Climate Change, up until now.
Interestingly, the radar altimeter data which seemed to show the effect are a bit in dispute. It is a very tough measurement to make, because the changes are so small. Something like 70 ice cores taken over the whole of that area showed no change in snowfall over the last 50 years at all. It seems that a compensating mechanism may not be taking place. It’s still an unresolved issue.
Now consider, what would be the consequence of that continuing discharge running its course? The Amundsen Sea Embayment is a very difficult area to reach, and indeed it was not until Carl Herb at the National Science Foundation deployed some C130s to put in major fuel dumps that the British Antarctic Survey and the University of Texas managed to get into the area a couple of years ago. They flew 30,000 kilometers of flight lines with two Twin Otters with ice penetrating radars on board and produced a map of the underside of the ice sheet. This showed that the ice accessible for discharge is equivalent to a one and a half meter, mean sea level rise. Of course, the whole of the West Antarctic ice sheet would raise sea level by 5 meters if it melted. The area that is currently discharging; if it continued to do so, it would raise sea level by one and a half meters or thereabouts.
How quickly could that happen? The discharge that we are seeing at present is of the order of a half a millimeter a year, sea level equivalent. Even if that accelerated, even if that doubled, we would still be talking about 1,000 years before that one and a half meters was delivered, all things being equal. The ice sheet model simply can not tell us, because the ice dynamics is not properly included, and there are even numerical issues about modeling the ice sheet near the ground inline.
Just looking at what is going on at present: there is what appears to be a slow acceleration of sea level rise that has been running at about 1.8 millimeters a year during the last century (Figure 6.14). It is now about 3 millimeters a year.
If we look at what has happened since the last ice age, there was a 9,000-year sustained period where sea level was rising at about one meter per century, and there were a couple of bursts that were substantially faster than that (Figure 6.15). Of course, sea level was stable for about 3,000 years. Whether or not the present configuration of ice sheets could deliver a meter per century or whether, because the warming that we are imposing, is at a rate 100 times faster than anything in the natural system—or at least if it follows the rate at which we are injecting carbon dioxide into
the atmosphere—is an open question. Just how quickly could this happen?
A few years ago, if I had discussed the flooding of London, I would have been concerned to be accused of alarmism. Increasingly these days, serious people are taking the threat very seriously.
INTERNATIONAL POLAR YEAR
In order to try and sharpen up on the answers to some of these questions, such as how much, how quickly, what is going on, the International Polar Year was established by ICSU and the World Meteorological Organisation, encompassing a wide variety of sciences.
It involves 63 nations, 50,000 individual participants, and some 229 projects, of which 170 are science based and 58 are for education and outreach. It has resulted in more than $300 million in new funding worldwide. IPY has a complex structure covering the Arctic, the Antarctic, land, ocean, atmosphere, ice,
people, and so on. As is to be expected, it has a major space component.
One of the most ambitious projects being undertaken is called GIIPSY, the Global Interagency IPY Polar Snapshot Year. This is an attempt to get a comprehensive series of “snapshots” by planning and synchronizing IPY satellite acquisition data requests, ultimately resulting from approved IPY projects.
Figure 6.17 provides an indication of the satellites that can be brought to bear to study the polar regions. Not shown is the CryoSat mission, scheduled for launch in 2009, which will provide a new view on the cryosphere. IPY set itself the task of leaving a legacy that changes the way polar science is executed. GIIPSY will be a major contribution to this task.
The martian year is 1.9 times that of Earth. Its atmosphere has only one percent of the surface pressure of Earth’s, is 95 percent carbon dioxide, and includes trace amounts of oxygen and water. Surface temperature ranges, from minus 143 degrees centigrade in the depths of the martian polar winter through to as much as 20, maybe even 25, degrees centigrade in the hot summer, with a mean of about minus 63 degrees centigrade. Clouds, fog, and frost all exist on the planet as do surface winds of about 400 kilometers an hour. There is also evidence (relic water flows) that there was free-flowing water on the surface of the planet in the past. Interesting questions therefore abound such as What was Mars like before? and What caused it to change? Mars has an obliquity very similar to Earth. If obliquity is important then, that matching between the two planets simplifies our understanding or our ability to understand.
Given the above, a lot of the Mars science has to do with “following the water” and also “following the carbon dioxide.” Indeed, it is interesting to note that the north polar cap, about 1,000 kilometers in diameter, has substantial water ice, up to 1.8 kilometers thick; with the seasonal carbon dioxide frost there, up to a meter thick.
In Figure 6.19 you can see in these two images of the north polar cap taken one martian year apart, a very substantial difference from one season to another, or if you like, one annual cycle to another. It is also apparent that the dust cover is critical, because blowing dust onto the ice, whether it’s carbon dioxide or water, changes its albedo, and that will change its ability to absorb or reflect heat.
Interestingly, there are quite substantial differences between the north polar cap and the south polar cap. They have quite different long-term histories, and the morphological features of their ice are substantially different.
In Figure 6.20, note the so-called “Swiss Cheese” appearance of the south polar cap and the very pitted nature of the north polar cap.
The south polar cap possesses water ice (Figure 6.21) plus a substantial carbon dioxide ice that seems permanent, although it varies in thickness. It is currently estimated that there is sufficient ice (water ice) in this polar cap that, if you melted it all, would cover the planet to a depth of 11 meters, i.e., a very substantial store of ice.
A number of spacecraft have provided different sorts of data concerning the martian polar regions. For instance we have MARSIS data, from the subsurface radar sounder on the Mars Express spacecraft which has allowed us to measure the thickness
of these polar caps and indeed map that thickness (Figure 6.22).
So, what can we learn from such data? Obviously trying to understand the behavior of Mars itself is a scientific challenge. However, given that there is currently no evidence of biological activity, we are then talking mainly about dynamics and physics.
The next major spacecraft event will be arrival of the Phoenix spacecraft a little bit later this year (May 25, 2008). It is actually scheduled to land on the edge of the northern polar ice cap and start to perform some tests, drilling down into the ice.
After the Apollo program ended, the United States rather lost interest in the Moon. The Russians completed their initial program of lunar rovers, orbiters,
landers, and sample return missions in 1976. There was then an extended hiatus in lunar missions.
The 1990s saw a rekindled interest, and missions have been flown by the United States, Japan, the European Space Agency, and recently by China and India. These last two nations have used such missions to demonstrate technological skills and capabilities.
NASA’s Lunar Reconnaissance Orbiter will begin to tell us a little bit more about the Moon, including whether or not water ice really does exist in shaded spots at the poles, and if so, how much there is.1 It will also be looking for safe landing sites for future crewed missions, locating potential resources, characterizing the radiation environment, and demonstrating new technologies.
Coming back to Earth, we need to remember that, in spite of all the rhetoric, carbon dioxide emissions into the atmosphere are firmly on, or indeed accelerating above, the business as usual curve. If we do not take action to stabilize these emissions at acceptable levels, temperature-projection models indicate the potential for a severe future a century from now and a planet which is completely transformed, with major social implications.
This is not the whole story. The global climate system is a complex interconnected non-linear system, capable of going through major reorganizations. A case in point: there is a 50 degrees centigrade temperature differential between the equator and the poles, and that, combined with the angular momentum effects or coriolus effects of being on a rotating planet, is what determines the flows of the fluids, the atmosphere, and the ocean. It’s interesting to note that because of the amplification of warming at the poles (by a factor of 2 or more), at some point, one would reach a situation where that temperature differential has been very substantially changed. Whether there would be a major non-linear reorganization, is something about which we need to be a little concerned.
The image in Figure 6.24 was taken from lunar orbit by the Japanese SELENE spacecraft. It was images of Earth such as this one that arguably crystallized in humanities mind the finite nature of the planet, its limited resources and the need to take care of it. This image, I think, should also inspire us to consider how serious the prospect of global climate change really is. To quote Socrates (ca. 450 B.C.): “Man must rise above Earth—to the top of the atmosphere and beyond—for only thus will he fully understand the world in which he lives.”