Less ice and snow, more rain and thaw: The weather forecast for Siberia, Alaska and Greenland is now like this, after all the Arctic is getting warmer. In the end, the wet ends up in the ocean faster and dilutes its saline solution. And maybe the sea will even make it a little cooler again.
Does anyone remember Wolfgang Emmerich's catastrophe scenario in "The day after tomorrow" when the Gulf Stream dried up and Europe and North America froze into ice palaces? Even if this plot was unrestrainedly exaggerated, it still contained a grain of truth: climatologists fear that global warming will actually cause the "Gulf Stream" circulating pump, which ensures ice-free ports beyond the Arctic Circle, to run out. Because the energy supplier from the tropics is part of the large-scale so-called thermohaline circulation, which transports warm water to the north-east. After cooling, this s alt-rich current plunges into the deep sea off Greenland as a huge waterfall, pulling new material from low latitudes onto the surface.
However, the system begins to stutter or possibly succumb completely when the salinity of the North Atlantic is diluted by fresh water to such an extent that the concentration of the current necessary for the sinking can no longer be achieved. In the past, this happened several times during the Heinrich events, when at the end of the ice ages exorbitant meltwater lakes from North America flowed into the Atlantic and thus indirectly delayed the rewarming of northern Europe or Canada - the warming Gulf Stream then turned further south than today.
The sweetening of the North Atlantic and the adjacent Arctic Ocean is currently taking place more slowly, the extent of which Bruce Peterson from the Marine Biological Laboratory in Woods Hole and his colleagues have now assessed. In the Arctic, s alt-free or at least low-s alt water comes from sources of different content, as the researchers found out. Between 1965 and 1995, the melting glaciers of Greenland and Svalbard, for example, provided around 2000 cubic kilometers of fresh water, a comparatively meager contribution to the corresponding total volume around the North Pole - but with an increasing influence since the mid-1990s.
The region's rapidly retreating sea ice contributed significantly more, which has lost an average of a tenth of its former area since 1978 – and up to a fifth in summer. In addition, the ice sheet thinned by between 15 and 40 percent, adding another 15,000 cubic kilometers of fresh water to the sea. Ultimately, the greatest contribution came from increased precipitation over land and sea in the northern latitudes – a consequence of rising temperatures that is not immediately apparent compared to melting glaciers and disappearing pack ice.
Changed air pressure conditions over the ocean in the three evaluated decades allowed moist air currents to penetrate further north than was previously the case, so that the clouds now shed their rain- or snow-heavy load more frequently in Siberia, northern Canada or northern Scandinavia. In addition, the arctic and subarctic regions warmed up significantly more than the rest of the planet; the warmer air, in turn, absorbs more moisture, which also increases precipitation levels. Either directly or via rivers such as the Lena, Ob and Yenisei or Yukon, the rain ultimately ends up in the sea: a total of 20,000 cubic meters.
Most of the fresh water then temporarily collects in the Arctic Ocean basin. Only when the so-called North Atlantic Oscillation (NOA) - the difference in air pressure between the Icelandic low and the Azores high - is particularly pronounced and an extra stiff breeze blows from the west, does it slosh into the North Atlantic. This exchange was clearly pronounced around the 1970s, but NOA is currently in a somewhat weaker phase again, which reduces the water exchange into the Atlantic. Based on their data, however, the researchers predict that revitalization will soon take place and with it another large freshwater surge southwards.
Perhaps the inflowing and relatively cool dew and rain water also contributed to the fact that the North Atlantic did not continue to heat up in 2003 and 2005, despite the very high air temperatures worldwide, but cooled down. Between 1955 and at least 1998, the seas warmed up by around 0.04 degrees Celsius on a global average - measured down to a depth of 3000 meters: in the layers near the surface the increase was significantly greater at 0.5 to 1 degree Celsius. In contrast, in 2003 and 2005 the top 750 meters of the world's oceans released about a fifth of the heat energy gained in the previous decades back into the atmosphere, according to measurements by researchers led by John Lyman of the US National Oceanic and Atmospheric Administration (NOAA) .
However, the researchers are not yet able to fully explain why the development of the sea bodies in these two years ran counter to the increasing air temperatures. They obtain their data in part from around 2,500 diving buoys and floating buoys floating on the sea, which can sink to great depths in order to record the salinity and temperatures there. However, these are unevenly distributed across the seven seas, so that a simple shift of warm water bodies to less densely populated regions could skew the results.
Possibly slightly changed ocean currents, the increased rise of cold deep water - such as west of Peru or Namibia - or the aforementioned cool freshwater increase from rivers and drift ice also play a role. Most likely, however, the researchers think a temporary cooling as a side effect of the warming. It sounds paradoxical, but it's not: Rising temperatures are driving evaporation, resulting in a net energy loss for the ocean and also promoting cloud formation, which has increased by one to two percent since 1999, according to analysis by the International Satellite Cloud Climatology Project. However, more clouds block the sun's rays and can delay or temporarily block direct heating, which turns the thermostat down in the water.
There is another reason why this argument is unsuitable for climate change skeptics, because the cooling should actually have led to a drop in sea level by an average of two millimeters: Just as heating water expands, cooling water should contract again. The opposite was the case, however, satellites measured a continuous increase in levels in all the years between 1993 and 2005 - and 2003 and 2005 were no exception. The melting glaciers in Antarctica and especially in Greenland may well be pumping more freshwater into the seas than is known, thus closing the loop for the time being.