Speaking of “OMG the ice is melting”, it is difficult to keep up with the flood (sorry) of data and analysis of the Earth’s ice and sea.
While I can glibly assert that the ice is melting and the seas are rising, the actual scientific picture is quite complicated. Let’s glance at some recent findings.
From space, we can observe lots of things, including ice shelves and bergs. NASA released imagery from MODIS instrument on the Terra satellite which shows a large berg that broke off the Nansen Ice Shelf in Antarctica last week. This is the end of summer there, so the ice would have been melting for a few of months.
There is actually imagery from past years, in which we can see the ice grow and then break off over several years.
It is important to note that this “berging” is not necessarily evidence that the ice sheet is shrinking there. Ice sheets build up and berg off all the time. But it does show that we can watch developments closely, and in near real time.
From space can also inventory seasonal ice coverage, which NASA has done since 1979. This year the “annual peak Arctic sea ice” was the smallest area since these data have been collected. The data is from a Special Sensor Microwave/Imagers on defense satellites, which can map ice and water.
“The low sea ice maximum coincides with record high air temperatures in December, January, and February in the Arctic and around the planet.”
This shrinkage continues a steady trend since at least 1979. There is a nice animation here.
There seems little question that glaciers and sea ice are melting in many places, including Greenland and the Arctic. This meltwater will surely end up in the ocean, and sea levels will rise.
But what is driving this trend?
An international team led by John A. Church and Ben Marzeion have presented a careful study based on computational simulations that consider several sources of climate change, including natural forces (e.g. fluctuations in solar influx), greenhouse gasses, and aerosols (e.g., dust and soot).[1] Based on physical models, They use historical data to model the contribution of each of these factors. The resulting computed sea levels can be compared to actual measured levels.
Their conclusion is clear. Sea level rise was primarily driven by natural variation in the first years of the twentieth century. But around 1970, around 70% of sea level change was due to anthropogenic factors, primarily greenhouse gasses.
This conclusion is not especially surprising. The important thing, I think, is that their work is comprehensive and very careful. It is a really nice illustration of how to use computation and data to try to understand the Earth’s climate, uncertainty and all. Very nice work, and I’m sure it will be carefully evaluated and built on by other investigators.
Together, these reports show the high quality, very careful study and hard work that goes into serious studies of the Earth’s climate, and the role that remote sensing plays.
- Aimee B. A. Slangen, John A. Church, Cecile Agosta, Xavier Fettweis, Ben Marzeion, and Kristin Richter, Anthropogenic forcing dominates global mean sea-level rise since 1970. Nature Clim. Change, advance online publication 04/11/online 2016. http://dx.doi.org/10.1038/nclimate2991
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