Concentrations of carbon dioxide (CO2) are rising rapidly in the atmosphere, due to the burning of fossil fuels and deforestation. About 25% of the extra CO2 added to the atmosphere through human activities is being absorbed by the oceans.
When atmospheric CO2 dissolves in seawater, it first forms carbonic acid and triggers a cascade of other chemical changes. The concentrations of hydrogen ions increase and carbonate ions decline. In fact, the concentrations of hydrogen ions have already increased by 30% in the seawater compared with pre-industrial times 250 years ago, increasing the level of ocean acidity. This change in the seawater chemistry is called “Ocean Acidification”.
The surface ocean pH has declined by about 0.1 so far, and is predicted to further decline by 0.2–0.4 by the end of this century, depending on future CO2 emissions. Geological records show that the present rate of change in the seawater chemistry is 10 to 100 times faster than it has been for many millions of years.
Although some scientists had recognised more than 50 years ago that rising CO2 concentrations will affect seawater chemistry and cause ocean acidification, this phenomenon has only recently emerged as one of the big knowledge gaps in marine science, and has now become a global research priority.
There is still very little known about how the ongoing changes in the seawater chemistry will affect marine ecosystems. Experimental studies suggest ocean acidification will profoundly affect the physiology and behaviour of some marine organisms. For example, ocean acidification makes it harder for some marine animals to form their shells and skeletons, but it may make it easier for some marine plants to do photosynthesis. A lower pH in the seawater may even lead to behavioural changes in fishes and invertebrates such as a reduced ability to recognise and avoid predators. It is unclear how natural marine ecosystems, such as coral reefs, will respond to such profound global changes.
“We have found that ocean acidification will select large boulder-like coral, over branching (bushy) corals that are the home of many species like fish, crabs, shrimps and sea stars. As a result, ocean acidification has a domino effect: as the habitat structure decreases, the animals that live and hide in their nooks and crannies find it far harder to survive, simply because they cannot hide from predators,”
Airborne surveys of southern Alaska have helped scientists get a better handle on where ice is being lost from this heavily glaciated region. Melting ice from Alaskan glaciers is estimated to be one of the main contributors to global sea level rise.
The maps above show changes to glaciers between 1994 and 2013 in southern Alaska, as well as parts of Canada’s Yukon Territory and British Columbia. Specifically, the maps show the region’s mass balance—the difference between the ice each glacier has gained and lost each year. The change is shown in meters of meltwater equivalent; that is, the depth of water that would result if that ice were melted. In general, red colors show where glaciers have thinned, and blue colors are where they have thickened. The bottom image shows a close-up view of the Wrangell and St. Elias mountain ranges.
Scientists estimated the mass balance from changes in the surface elevation of 116 glaciers, which were observed at roughly the same time every year with lidar instruments mounted on aircraft. They determined that the region lost about 75 billion tons of ice per year over a 19-year period. That’s about 30 percent of the amount of ice thought to be lost each year from the Greenland ice sheet. The results were published in June 2015 in Geophysical Research Letters.
As the maps show, not all glaciers are losing ice equally. “From glacier to glacier, there is a large amount of variability in how each is responding to climate,” said Chris Larsen, a researcher at University of Alaska at Fairbanks and lead author of the paper.
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The West Antarctic Ice Sheet is a candidate to have supplied about 3 meters of that sea level rise. Unfortunately, evidence of its history is hard to come by, as the regrowth of the ice sheet destroyed some of it and now conceals even more. Sediment cores show the ice sheet shrank drastically in the past, but it’s unclear when in the past.
A new study led by University of Washington researcher Eric Steig applies a suitably clever work-around to get at the ice sheet’s history. It’s based on the impact a collapse of the ice sheet (if it shrank down to a small remnant) would have had on local atmospheric circulation. Losing the ice sheet, after all, is like deflating an entire landscape. There are good reasons to expect that thickening the atmosphere by lowering the surface in that area would have consequences.