CLIMATE BRIEFS

Satellites Observe Longer Melt Season in the Arctic

     Though it's true that Arctic lands and sea ice have cooled in the winter, they have warmed in spring, summer, and fall, reports J.C. Comiso in the Journal of Climate1. Studying 20 years of satellite observations, he concluded that North America warmed the most at a rate of 1.1°C per decade, while Europe and Asia warmed at 0.4°C and sea ice at one-third °C per decade. The trend in Greenland was insignificant. Overall, earlier melting in spring, warmer summers, and later freezeup in fall have lengthened the season of ice melt by 9 days over sea ice, and as much as 17 days in North America, in each decade.

     Satellite-observed temperatures differed from observations taken on the surface by about 1.5 to 3°C, but Comiso believes that the satellite data are internally consistent and can detect trends well. The trend over the last 20 years is some eight times larger than the trend over the last 100 years (the latter from surface reports, of course). The recent warming trend, if sustained, would profoundly reduce the volume of sea ice, he concludes, with a further impact on global climate.

1. “Warming Trends in the Arctic from Clear Sky Satellite Observations,” by J.C. Comiso (2003), Journal of Climate, vol. 16, no. 21, 3498-3510.

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Large Cities Boost Rainfall Downwind: Focus on Houston

     In the 1970's, evidence arose that major cities cause more rainfall to fall on land downwind of the city. That led to a field experiment called “METROMEX” in the St. Louis area, which confirmed that urban effects of St. Louis cause from 5% to 25% more precipitation in the summer, in the city and up to 75 km downwind of it. Now J. Marshall Shepherd of the NASA Goddard Space Flight Center, and S. J. Burian, used a radar on board a satellite (the Tropical Rainfall Measuring Mission - TRMM) to detect whether Houston affects regional rainfall. Indeed, elevated rainfall was found in Houston and downwind of it (usually northeast of the city), in the summer season and in the annual average.

     The authors presented evidence that the urban center is causing the increase, not just the sea breeze. They say that Houston's urban “heat island” sets up local winds that interact with the sea breezes in such a way that thunderstorms are favored over the city, and downwind of it. Their results are supported by maps of lightning location published by Orville3 in 2001.

     The authors include a movie in their paper in Earth Interactions 4.

 

3. “Enhancement of cloud-to-ground lightning over Houston, Texas,” by Richard E. Orville and 9 others (2001), Geophysical Research Letters, v. 28, no.13, 2597-2600.

4. “Detection of an Urban-Induced Rainfall Anomalies in a Major Coastal City,” by J. Marshall Shepherd and Steven J. Burian (2003), Earth Interactions (online), vol. 7, no. 4.

 

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Tropical Sea Water Saltier, Polar Water Fresher

     Three oceanographers led by Ruth Curry report in Nature 2 that waters in the upper half of the Atlantic Ocean have become more saline in the Tropics and subtropics. Since salt is nearly conserved in the Oceans, and Tropical Atlantic waters are already saltier than anywhere else, salt cannot be arriving from elsewhere. They concluded the rate of evaporation has increased 5 to 10% from the warm water over the 40 year of the study. There was “an unambiguous connection between the water masses that became more saline and those that warmed”, Curry asserted, which points to long-term warming of ocean waters as the cause. In the same 40 years, polar waters became less salty in both the North and the South Atlantic. The probable cause of water freshening in the polar regions was greater inflow of fresh water from rivers and rainfall and more melting of ice.

     Fresh water was lost in the Tropics and added in polar regions, at a pace that exceeded the ability of the Ocean circulation to compensate. Taken together, the accelerated evaporation in the Tropics and the greater precipitation and ice melt in polar latitudes suggest that the global water cycle has speeded up. That cycle is one of the least understood parts of the climate system; and climate models differ in their handling of their budgets of fresh water.

2. “A change in the freshwater balance of the Atlantic Ocean over the past four decades”, by Ruth Curry, B. Dickson, and I. Yashayaev (2003), Nature v. 426, 826, 18 Dec 2003.

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Northern Forests Remove Half of Missing CO2

     A new inventory of forests has doubled the estimated size of the “carbon sink” where carbon is believed to accumulate in the world. People have been searching for a “missing sink” of carbon in the global carbon budget for a long time. Although combustion of fossil fuels and deforestation emits 8 billion metric tons of carbon into the

atmosphere as CO2, only about 3 billion tons are showing up in the atmosphere. Of the other 5 billion metric tons (the “missing sink”), about half is absorbed by the oceans, and half is taken up by growing plants on land.

     In an article in Climatic Change 5, Jari Liski inventoried the growth and removal of wood in 55 countries that have about one-half of the world’s forest area. These temperate and polar forests take up from 0.7 to 1.1 billion metric tons of carbon per year, an estimate that is one half of the missing sink, and is nearly twice as large as the estimates made for the 1980's.


     The carbon sink in northern forests is larger than expected because real tree growth has increased in all regions and tree cutting diminished in Russia, the authors conclude.

5. “Increased Carbon Sink in Temperate and Boreal Forests,” by Jari Liski and 5 others (2003), Climatic Change, vol. 61, 89–99.

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Oceans May Soak Up Some Future Warmth

     Predictions of future global temperature change are difficult, because there are a number of factors that influence global climates, and because the sensitivity of Earth's climate to these factors is not well known. One major player is the ability of the Oceans to absorb heat and store it in deep waters. Sun and Hansen reported results of their Ocean-Atmosphere simulations 6, , which suggest that the atmosphere will warm about +0.5 °C over the next 50 years, only two-thirds as much as they reported two years ago.

     Their new experiments represent the Oceans in three different “models”, each of which they linked to their best atmospheric climate model. One model had a “passive” ocean; another “active” model mixed water down to a depth of one km; while a third “dynamic” model, considered the most realistic, mixed heat and water to great depths. They wanted to learn how representation of the Oceans affects the warming of the atmosphere, present and future.

     The ability of Oceans to store heat in deep waters slows the warming of the atmosphere, as expected. This “has both good and bad sides”, the authors say. It provides time to better monitor how the Earth is changing, and a “longer period to act before the largest consequences of global warming occur”. The bad side is more “warming in the pipeline but not yet realized”.

     Secondly, they find that the Earth-Ocean climate system was “out of balance” with incoming solar energy even in 1950, but is now more than three times more out of balance as then. One can estimate this balance by measuring the net flux of energy into the Earth (in units of Watts per square meter). At first they assumed the Earth was in balance in 1950, but their work suggests an imbalance of 0.2 units in 1950, of 0.75 units now, and of 1.3 units in 2050 under their assumptions about the future.

     Finally, more complete measurements of the Oceans are needed, at greater depths and in more remote areas. Roger Pielke made the same point in discussing the effect of land use on climate (see the Spring 2003 Climate Science Forum).

6. “Climate simulations for 1951-2050 with a Coupled Atmospheric-Ocean Model,” by Shan Sun and James Hansen (2003), Journal of Climate, 1 Sep 2003, vol. 16, 2807-2825.

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