Friday, October 05, 2007


Reply to Lockwood and Froehlich - The persistent role of the Sun in climate Forcing

In a recent paper (ref. [1]) Mike Lockwood and Claus Froehlich have argued that recent trends in solar climate forcing have been in the wrong direction to account for "the observed rapid rise in global mean temperatures". These authors accept that "there is considerable evidence for solar influence on Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century." But they argue that this historical link between the Sun and climate came to an end about 20 years ago. Here we rebut their argument comprehensively. [...]

By Lockwood and Froehlich's own data, solar magnetic activity is still high compared with 100 years ago. As to when the recent easing of activity began, counts of cosmic-ray muons at low altitudes were historically low when the muon record-keeping ended in the early 1990s (ref. [7]). That implies an increase in relevant solar magnetic activity continuing till that time. A scarcity of muons can be linked to elevated global temperatures by a reduction in low cloud cover (ref. [8]) and low cloudiness was indeed at a minimum around 1992-93. By other solar indicators, like those cited by Lockwood and Froehlich, the minimum muon counts may well be a little higher in the current solar cycles. That would explain the pause in global warming evident in our Table as well as in Lockwood and Fr”hlich's own Fig. 1e.

That would explain the pause in global warming evident especially in the ocean (Fig. 1) and the troposphere (Fig. 3). The continuing rapid increase in carbon dioxide concentrations during the past 10-15 years has apparently been unable to overrule the flattening of the temperature trend as a result of the Sun settling at a high, but no longer increasing, level of magnetic activity. Contrary to the argument of Lockwood and Froehlich, the Sun still appears to be the main forcing agent in global climate change.



Discussing: Scileppi, E. and Donnelly, J.P. 2007. Sedimentary evidence of hurricane strikes in western Long Island, New York. Geochemistry, Geophysics, Geosystems 8: 10.1029/2006GC001463.


The authors note that "when a hurricane makes landfall, waves and storm surge can overtop coastal barriers, depositing sandy overwash fans on backbarrier salt marshes and tidal flats," and that long-term records of hurricane activity are thus formed "as organic-rich sediments accumulate over storm-induced deposits, preserving coarse overwash layers."

What was done

Scileppe and Donnelly refined and lengthened the hurricane record of the New York City area by first calibrating the sedimentary record of surrounding backbarrier environments to documented hurricanes - including the hurricanes of 1893, 1821, 1788 and 1693 - and then extracting several thousand additional years of hurricane history from this important sedimentary archive.

What was learned

The two researchers report that "alternating periods of quiescent conditions and frequent hurricane landfall are recorded in the sedimentary record and likely indicate that climate conditions may have modulated hurricane activity on millennial timescales." Of special interest in this regard, as they describe it, is the fact that "several major hurricanes occur in the western Long Island record during the latter part of the Little Ice Age (~1550-1850 AD) when sea surface temperatures were generally colder than present," but that "no major hurricanes have impacted this area since 1893," when the earth was in the initial stages of its transition from the Little Ice Age to the Modern Warm Period.

What it means

Noting that (1) Emanuel (2005) and Webster et al. (2005) have produced analyses that suggest that "cooler climate conditions in the past may have resulted in fewer strong hurricanes," but that (2) their own findings suggest just the opposite, Scileppe and Donnelly conclude that "other climate phenomena, such as atmospheric circulation, may have been favorable for intense hurricane development despite lower sea surface temperatures," prior to the development of the Modern Warm Period. Perhaps, therefore, we have much-maligned global warming to thank for the complete absence of major hurricanes in the vicinity of New York City over the past 114 years.

Extract from the original paper below:

Sedimentary evidence of hurricane strikes in western Long Island, New York

By Elyse Scileppi et al.

[1] Evidence of historical landfalling hurricanes and prehistoric storms has been recovered from backbarrier environments in the New York City area. Overwash deposits correlate with landfalls of the most intense documented hurricanes in the area, including the hurricanes of 1893, 1821, 1788, and 1693 A.D. There is little evidence of intense hurricane landfalls in the region for several hundred years prior to the late 17th century A.D. The apparent increase in intense hurricane landfalls around 300 years ago occurs during the latter half of the Little Ice Age, a time of lower tropical sea surface temperatures. Multiple washovers laid down between 2200 and 900 cal yr B.P. suggest an interval of frequent intense hurricane landfalls in the region. Our results provide preliminary evidence that fluctuations in intense hurricane landfall in the northeastern United States were roughly synchronous with hurricane landfall fluctuations observed for the Caribbean and Gulf Coast, suggesting North Atlantic-wide changes in hurricane activity. [...]

7. Conclusions

[57] A record of hurricane landfall is preserved in backbarrier marshes of the New York City area. This region has been impacted by numerous strong hurricanes during the last 3500 years even though no major hurricanes have impacted this area since 1893. Widespread storm-induced deposits preserved in backbarrier sediments indicate the likely preservation of washovers corresponding to the 1893, 1821, 1788, and 1693 hurricanes. The 1821 (possibly 1788) hurricane caused region-wide overwash from western Long Island to southern New Jersey. A localized overwash deposit is recorded at one marsh that corresponds to Hurricane Gloria which occurred in 1985 with a moderate storm surge. Coarse layers preserved likely correspond to prehistoric storms and more work is necessary to determine their spatial consistency across study sites. An apparent lull in intense hurricane landfalls prior to the historic record is evident in cores from the three study sites as well as southern New Jersey, indicating that a period of infrequent hurricane landfall may have occurred in the region between about 900 cal yr B.P. and 250 cal yr B.P. (i.e., 1693 A.D.). Despite significantly cooler than modern SSTs in the Atlantic during the latter half of the Little Ice Age, the frequency of intense hurricane landfalls increased during this time. The relatively quiescent interval spans times with relatively cool and warm Atlantic SSTs.

[58] The longest record from western Long Island reveals intervals of more frequent overwash deposition punctuating intervals of quiescent backbarrier sedimentation. Alternating periods of quiescent conditions and frequent hurricane landfall are recorded in the sedimentary record and likely indicate that climate conditions may have modulated hurricane activity on millennial timescales.

Although additional records are necessary to test this hypothesis, the possible synchroneity of increased storm activity in western Long Island (2200-900 cal yr B.P. and pre 2800 cal yr B.P.) and the northern Gulf Coast ( 3650-930 cal yr B.P.) suggests that landfall patterns may be caused by overall increases in storm frequency and are not simply due to changing hurricane tracks.



"Given this state of affairs, projections of changes in [tropical cyclone] intensity due to future global warming must be approached cautiously." This is the concluding sentence of a just-published article by University of Wisconsin-Milwaukee's Kyle Swanson in which he carefully examined the historical relationship between sea surface temperatures and tropical cyclone intensity in the Atlantic and western Pacific ocean. Swanson justified his research efforts, well summarizing the current state of things (including references):

"Recent studies have found an apparent increase in the proportion and number of tropical cyclones (TCs) that become intense [Webster et al., 2005] along with links of this increase to positive sea surface temperature anomalies [Emanuel, 2005; Hoyos et al., 2006] and possibly global warming [Trenberth, 2005]. However, the sensitivity of TCs to changes in sea surface temperature (SST) remains controversial [Landsea et al., 2006; Shapiro and Goldenberg, 1998], as modeling and theoretical studies suggest only small changes to TC intensities given the observed 0.5§C SST warming that has occurred since the 1970s [Emanuel, 1988; Knutson et al., 2001]). Further, satellite reanalysis suggests no increase in the fraction of intense TCs outside the North Atlantic basin [Kossin et al., 2007]. Trends in TC intensity are difficult to discern, as statistics are inherently noisy due to fluctuating storm numbers and life spans. As the theory underlying TC intensities specifically predicts only the maximum potential intensity, it is necessary to control for these other factors if the response of the TC intensity to changes in SST is to be understood."

In looking for the primary drivers of tropical cyclone intensity, Swanson found that tropical cyclones do not always react the same way to changes in local sea surface temperatures (SSTs). During some periods, like the mid-1970s through the present, an increase in the percentage of stronger hurricanes has accompanied rising SSTs, but during other earlier periods, the apparent relationship was not so clear. In fact, overall, Swanson found no statistically significant correlation between SSTs and average tropical cyclone intensity in either ocean basin during the 1950 to 2005 period of his study.

Consequently, here is what Swanson had to say about recent papers claiming to have found an definitive link between rising SSTs, tropical cyclone intensity (and anthropogenic global warming): "[T]he period 1975-2004 examined by Webster et al. [2005] is fortuitous; it captures the minimum of [tropical cyclone, TC] intensities during the 1970s and the subsequent increase in TC intensities. However, the post-1975 upward intensity trend over this period does not appear to mark a fundamental shift in TC intensity behavior; this behavior is still within the upper bound set during the 1950s in both the NATL and WNPAC basins."

Instead of a relationship with local SST variability, Swanson found that tropical cyclone intensity was much more closely linked to local SST anomalies-that is, the difference between the SST in the primary tropical cyclone formation regions in the Atlantic and the western Pacific, and that of the average SST in the tropics as a whole. For instance, during the times when the central tropical Atlantic SST were higher than the average SST across the entire tropics, Atlantic hurricane activity and intensity levels were above normal, conversely, when the Atlantic SSTs were below the tropical average, hurricane activity was diminished. Figure 1, taken from Swanson (2007), depicts this relationship. Average hurricane intensity was as high in the 1950s and early 1960s as it has been recently (Figure 1b and c), despite the fact that SSTs were more than 0.5§C lower in the main cyclone development regions in the Atlantic and western Pacific in the 1950s than presently (Figure 1a). But, when compared to the average SST in all the world's tropical areas, the 1950s and early 1960s were relatively warm in the cyclone development regions in both the Atlantic and western Pacific.

Swanson suggests that this type of behavior "is consistent with the tendency for regions of anomalously warm SSTs to cannibalize moist convection in the tropics, most apparent in the global-scale reorganization of convective behavior that occurs during El Ni¤o events." In other words, warm pools of water, rather than uniformly warm water, are more conducive to firing up thunderstorm complexes that can lead to tropical cyclone formation.

Here is how Swanson sums up this finding, including its implication for predictions of global warming-induced changes to tropical cyclone intensities: "Finally, the apparent sensitivity of TC intensity to relative MDR SST anomalies is itself troublesome. How these relative SST anomalies will change under global warming scenarios is unclear, as modeling relative SST anomalies is a much more difficult task than modeling SST anomalies for the tropics as a whole. As such, it is unclear whether the coincident increase in MDR SST anomalies and relative MDR SST anomalies since the mid-1970s shown in [our Figure 1, above] will continue. Given this state of affairs, projections of changes in TC intensity due to future global warming must be approached cautiously."

Swanson's conclusions are similar to those recently reported by some folks working out of the University of Wisconsin's more westerly Madison campus. Jim Kossin and colleagues conducted a research project (for more details of the Kossin et al., study, see here) in which they carefully constructed a homogenous tropical cyclone dataset for all the world's ocean basins for the past 23 years. After examining their new record for trends, they concluded: "Using a homogeneous record, we were not able to corroborate the presence of upward trends in hurricane intensity over the past two decades in any basin other than the Atlantic. Since the Atlantic basin accounts for less than 15% of global hurricane activity, this result poses a challenge to hypotheses that directly relate globally increasing tropical SST to increases in long-term mean global hurricane intensity."

It certainly is beginning to seem that the more and more people look, the less and less they can find any clear relationship between rising SSTs and increased activity and intensity levels of tropical cyclones. Further, the lack of a clear understanding of the past and present relationships serves to cloud our ability to see into the future when many aspects of the tropical environment are projected to change, not simply sea surface temperatures (for more information about how these other projected changes may impede tropical cyclone development, see here).


Recent Rapid Decline in Arctic Sea Ice caused by Unusual Winds, says NASA

Post below lifted from Accuweather. See the original for links and graphics

In a news release from NASA Monday, a group of scientists have determined that unusual winds caused the rapid decline (23% loss) in winter perennial ice over the past two years in the northern hemisphere. This drastic reduction is the primary cause of this summer's fastest-ever sea ice retreat in recorded history which has lead to the smallest extent of total Arctic coverage on record.

According to the NASA study, the perennial ice shrunk by an area the size of Texas and California combined between the winter of 2005 and the winter of 2007. What they found was the Arctic Ocean north of Siberia and Alaska was dominated by thinner seasonal ice that melts faster compared to the thicker ice confined to the Arctic Ocean north of Canada. The thinner ice is more easily compressed and responds more quickly to being pushed out of the Arctic by winds.

"Unusual atmospheric conditions set up wind patterns that compressed the sea ice, loaded it into the Transpolar Drift Stream and then sped its flow out of the Arctic," said Son Nghiem of NASA's Jet Propulsion Laboratory and leader of the study. When that sea ice reached lower latitudes, it rapidly melted in the warmer waters.

What about these unusual wind patterns. Well, the article does not go into that too much, but I must believe some of this is due to changes in the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) which are large atmospheric circulations which have major impacts on the weather in certain parts of the world.

If you look at the two graphics below, you will notice that both the AO and the NAO have been predominately in the positive phase (red) between 1989-1995 and again from 1999 to current. The positive phase of the (AO) typically leads to milder than normal winters over Scandinavia and Siberia, while colder than normal conditions prevail across Greenland. The positive phase of the NAO again leads to colder conditions over Greenland, while much of the eastern U.S. is warmer than normal in general.

Now light is pollution

Here's your mission, should you choose to accept it: Look up in the sky after dark and count the stars you see. As part of a worldwide push to gauge how much light pollution is out there, a scientific organization is asking people to look for specific constellations during the next two weeks and share observations on the Internet.

You probably won't be able to see the Milky Way from your San Jose backyard, experts say - but this area is still better off than some when it comes to the artificial lighting that illuminates the sky and interferes with stargazing. It isn't as bright as Los Angeles, isn't as gaudy as Las Vegas, and is home to the Lick Observatory - which has worked closely with San Jose officials to ensure special streetlights are used here.

The stargazing effort, dubbed the Great World Wide Star Count, is open to anyone who has access to a computer and the World Wide Web. And while scientists hope it will help them map light pollution on a global scale, they do have another motive. "We want people to go outside and look up, to appreciate the night sky," said Dennis Ward, an educational technologist and astronomer with the University Corporation for Atmospheric Research in Boulder, Colo. The consortium of universities is organizing the event along with planetariums and scientific societies across the country.

Under perfect conditions - no moon, a clear sky and minimal light pollution - a stargazer should be able to see as many as 14,000 stars, Ward said. But in many major cities, where used car lots, shopping malls and football stadiums illuminate the night, often fewer than 150 are visible. "It's pretty bad," said Ben Burress, staff astronomer at Chabot Space & Science Center in Oakland. "People who live in cities tend to not think about the nighttime sky very much because they can't see it very well," he said. "You can't just walk outside and see something really enthralling."

That's a shame - and probably a major deterrent to kids developing an interest in the sciences these days, said Bob Gent president of the board of directors of the International Dark-Sky Association in Tucson, Ariz. When Gent was a young boy growing up in Phoenix, he could view a sky filled with thousands of objects, including his favorite, the Milky Way. Seeing the constellations made him curious about the universe and our solar system, and instilled in him a respect for the outdoors. "I became an astronomer for life by the time I was 5 years old," he said. "But now we've lost the heritage of dark skies in these big cities."

Light pollution has become a growing problem around the globe, fueled by urban sprawl and a growing population. Satellite images show much of the U.S. eastern seaboard is socked in by light pollution, as well as most large cities across the rest of the country. Among the problems caused by light pollution: Bright lights have interrupted the migratory patterns of birds and disoriented baby marine turtles. As a result, dozens of communities across the country have begun to enact ordinances aimed at reducing the glare. "It's not just astronomy impacted," Gent said. "It's a wildlife issue. It's the loss of the inspiration of the night sky. It's all those - while we're wasting energy."

When it comes to lighting, many city governments and private corporations have kept safety issues - and good business - squarely in mind, opting to keep streets, parking lots and store displays well-lit. While dark-sky advocates aren't calling for blackened city scapes, they do point out that some lighting is purely ornamental and unnecessary - think Las Vegas.

Despite the fact that the Bay Area has been densely populated for decades, it wasn't that long ago that you could drive a little ways and get a magnificent view of the sky, said Marni Berendsen, education project coordinator for the San Francisco-based Astronomical Society of the Pacific. During the early 1990s, for example, Berendsen would go to Mt. Diablo near Walnut Creek to see the Milky Way. No longer. "As the years go by and more and more houses are built at the base of the mountain," she said, "we're really starting to lose our dark sky up there."

San Jose is one community that has made great strides to reduce its light pollution - at least the kind that can obstruct observatory instruments. Of the city's 60,293 streetlights, an estimated 51,341 are low-pressure sodium lights, energy savers that give off a yellowish hue. They produce a light that researchers at the Lick Observatory can easily filter out. "They've really gone out of their way to help the observatory," said Burt Jones, assistant director of Lick. "Most cities are not like that."

Organizers of the Great World Wide Star Count wish more communities would take similar steps to turn down the lights. If their event becomes an annual one as they envision, scientists will be able to compare data from year to year and map the light-pollution changes. They hope their event will raise awareness of the issue, while shedding light on how poor the stargazing is from some urban centers. "That's one of our goals," Ward said, "to make people understand there could be so much more."



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