The ice is melting and the seas are rising. Lets define the cryosphere as the totality of the earth's ice. So the cryosphere includes glaciers which are freshwater and these are thick masses of ice on land form from snow accumulation over hundreds and thousands of years. There are two types of glaciers we could consider, mountain glaciers and ice sheets, which are massive glaciers that basically are only found in the Arctic and the Antarctic. Then there's also sea ice is another part of the cryosphere. That is frozen seawater that's floating on top of the ocean. So that is salt water as opposed to freshwater. Now, the thing about the cryosphere is that it really illustrates how our fate is tied to the natural world on a global scale. So what happens in remote mountains where there are glaciers and in the Arctic and the Antarctic with respect to the ice sheets will have a profound effect on all of us. So let's take a look at that. Okay. On this slide, let's just focus on the line as opposed to the bars. So what that's showing is the cumulative annual balance for 41 mountain glaciers. So what we're looking at here and this is from 1980 to 2012, so we're looking at how the mass of these glaciers has decreased over that period. This is the scale is in millimeters. So for the thickness of the glacier. So the decrease has been more than 18,000 millimeters or 18 meters over that roughly 30-plus-year period. Notice that the scale for the cumulative decrease is on the right sided y-axis and I'm sure many of you have seen slides like this. Here, we're looking at a particular mountain glacier, the South Cascade Glacier in the Washington Cascade Mountains. We're comparing what it looked like in 1928 with what it looked like in 2000 and the recession of the glacier is obvious. Now, let's turn to the ice sheets, and again these are massive glaciers. The Antarctic ice sheet which has actually three different ice sheets, I'll show that in a minute, is massive. and it contains roughly 200 feet of sea-level ice. Meaning that if the Antarctic ice sheet melted completely, sea level would rise by 200 feet. The Greenland ice sheet is much smaller, yet highly significant and it holds about 20 feet of sea level rise. So what have been the actual changes in ice sheet mass? So this is looking at roughly 1990 to the near present and we could see that for West Antarctica, there's been some decrease in mass. The sharpest decrease has been in Greenland. We saw earlier that that's because the greatest warming has been occurring in the Arctic. The East Antarctic Ice Sheet has been fairly stable over the last 30 so years, but the Antarctic Peninsula ice sheet has lost some mass. The previous slide showed data up to 2012, but the 2018 study extended these Data for Antarctica out to 2017. Notice that most of the overall Antarctica ice sheet mass loss is due to loss in West Antarctica. The investigators calculated the rate of Antarctic ice sheet mass loss and found it to be accelerating. For East Antarctica, mass was actually added in 1992 to 2011, whereas mass was lost in 2012 to 2017. I should point out that these data were the least certain of their results. For West Antarctica and the Antarctic Peninsula, the rate of ice sheet mass loss approximately doubled between 2012-2017 verses 1992 to 2011. For Antarctica overall, the rate of loss almost tripled. Since acceleration of Antarctic ice sheet mass loss translates into acceleration of sea level rise, this is an important warning sign. Now, let's turn to sea ice. Sea ice, as I said, is frozen seawater floating on top of the ocean and because it's floating on top of the ocean, it also does not raise sea level when it melts. There are concerns with regard to climate change for sea ice, which I'll describe a little later. So let's look at the trends in sea ice. You've probably read in the news a lot about the decrease that's been occurring in the Arctic sea ice. This slide shows from 1978 to 2017 the blue is Arctic and we could see the steady decline in the sea ice in the Arctic, whereas the red is the Antarctic where there's actually been an increase in sea ice up until around 2014 or so when we see this very sharp decline. Now, it's too early to tell whether that's an important trend or whether it's just an anomaly that we'll have to wait and see about that. So now, let's turn to sea-level rise. So sea level rises for two reasons, one is that water expands when it warms. So as I showed earlier, the oceans are absorbing heat, they're warming and therefore the oceans are expanding, that raises the sea levels. Also, sea level rises when glaciers or ice sheets melt water runs into the ocean. So let's take a look at first the cumulative Greenland ice mass loss and the result in sea level rise. This is from 1990 to 2012. So you can see that the loss in mass has been roughly 3,000 gigatons, that's billions of tons, it's a lot. That has resulted in a sea level rise of about eight millimeters. Same slide for Antarctica and since the Antarctic has not been warming nearly as fast as the Arctic has, we see that the mass loss in Antarctica has been less. It's been less than 2,000 gigatons resulting in a less than six millimeter sea level rise. However, as mentioned previously, a 2018 studies showed that Antarctic ice mass loss, and therefore a sea level rise resulting from this loss has been accelerating. So this slide shows from about 1990 to the near present the global mean sea level rise. So you can see that there's been a continuous trend of rising sea levels and that's been roughly 3.1 millimeters per year overall. This is from a combination of what I showed you in the two previous slides, the ice sheet melts in Antarctica and Greenland, also for melt of mountain glaciers which actually has contributed more at this point than has melt from Antarctica and the Arctic and plus the third factor is the expansion of the oceans due to the warming. The concern, of course which I'll get to later as well is that this sea level rise will accelerate.
