August 22, 2007

Ocean Circulation Slowdown: False Alarm

We are sure many of you remember headlines similar to these: “Global Warming to Cause Next Ice Age!” or “Global Warming to Send Europe into a Deep Freeze!” In fact, next time New England or Europe has a cold winter, we’ll guarantee that you’ll see them again. The idea behind this scare story (and the premise of the climatefright film The Day After Tomorrow) is that the ocean’s thermohaline circulation (which among other things modestly warms the winter climate of western Europe) slows down, or even worse stops, sending the climate into disarray—all because of anthropogenic global warming. In the case of The Day After Tomorrow, this circulation shut down led to a flash freeze of the planet, while more “reasonable” climate alarmists at least give it a couple of decades to turn Europe into the icebox. But, in reality, things just don’t seem to be headed that way at all.

The thermohaline circulation works something like this: strong solar heating and warm waters in the tropical Atlantic result in enhanced evaporation there, leaving the surface waters there saltier than the average ocean. These warm, salty waters are carried northward via the Gulf Stream and in the high latitudes they release the heat into the atmosphere, and subsequently cool. This cool, salty current of water becomes more dense than the less salty waters surrounding it and consequently sinks and flows back southward acting as a sort of pump that drives this major circulation system that circuitously winds its way though most of the world’s major oceans (see here for more details).

There are indications from paleoclimate studies that the thermohaline circulation has shut down in the past, causing “abrupt climate changes.” This has occurred when there was a large freshwater input into the North Atlantic—fresh water is less dense than saltier water, so this inhibits the sinking water in the north and effectively shuts down the pumping mechanism. The last time this happened was about 8,200 years ago, when the world was still emerging from the last ice age. As the ice sheet that covered North America melted, it formed a huge lake in central Canada (called Lake Agassiz) that contained more water than the combined Great Lakes do currently. Lake Agassiz was held back by an ice dam, that eventually disintegrated as the climate warmed, and virtually instantaneously dumped the lake’s contents through Hudson bay and into the North Atlantic Ocean. This fresh “meltwater pulse” apparently shut down the pump, which took a couple hundred years to get up and running again, during which time Greenland and Europe were considerably cooler then they were previous to this event. (See here for more details).

Note to all readers: today, there is no bigger-than-all-the-Great-Lakes-combined glacial meltwater lake in central Canada held back by an ice dam on the verge of collapse. But this hasn’t led folks to abandon the idea that human-induced climate warming may lead to an “abrupt climate change” caused by a shutdown of the ocean’s thermohaline circulation. The idea is that global warming will lead to a meltdown of the Northern Hemisphere’s last significant ice sheet remnant from the Ice Age—the one lying atop Greenland. The meltwater from Greenland ice sheet, together with a projected enhancement of high latitude precipitation will eventually provide a large enough input of fresh water to the subpolar North Atlantic ocean to slow and eventually halt the thermohalime circulation. Or so some climate models tell us.

And since climate models foretell of this occurrence, researchers have been pouring over data in search of any indications that it may be getting underway. Hints that they may have identified a thermohaline slowdown are met with glee by climate alarmists and trumpeted to the disaster–loving press.

For example, in December 2003, Ruth Curry, from Woods Hole Oceanographic Institution, and colleagues reported in Nature magazine that they had detected evidence that suggested the thermohaline circulation was slowing down. From data collected from the 1950s to the 1990s, they reported that the tropical Atlantic was growing saltier while the northern latitudes of the Atlantic were growing fresher and suggested that anthropogenic global warming was the probable cause. The Toronto Globe and Mail covered the findings such:

Atlantic’s salt balance poses threat, study says

The Toronto Globe and Mail, Dec. 18, 2003

The delicate salt balance of the Atlantic Ocean has altered so dramatically in the last four decades through global warming that it is changing the very heat-conduction mechanism of the ocean and stands to turn Northern Europe into a frigid zone.

The conclusions are from a study in the journal Nature that is to be published today. The study describes planet-scale changes in the regulatory function of the ocean that affect precipitation, evaporation, fresh-water cycles and climate.

“This has the potential to change the circulation of the ocean significantly in our lifetime,” said Ruth Curry of the Woods Hole Oceanographic Institution in Massachusetts, the study’s lead author.

Then, two years, later, in December 2005, Harry Bryden and colleagues published an article, again in Nature that seemed to show additional evidence of the thermohaline circulation’s long slow death march. They examined a series of ship transects of the Atlantic Ocean, the first in 1957 and the last in 2004, and declared that they had detected a circulation slowdown of about 30% which primarily had taken place during the past 10-15 years. Here is how U.K’s The Guardian began its coverage:

Alarm over dramatic weakening of Gulf Stream

The Guardian, Thursday December 1 2005

The powerful ocean current that bathes Britain and northern Europe in warm waters from the tropics has weakened dramatically in recent years, a consequence of global warming that could trigger more severe winters and cooler summers across the region, scientists warn today.

Researchers on a scientific expedition in the Atlantic Ocean measured the strength of the current between Africa and the east coast of America and found that the circulation has slowed by 30% since a previous expedition 12 years ago.

In the intervening time since then, the literature has been filled with articles that have benefited from having more and better data available and virtually all of them have been unable to verify a slowdown in the thermohaline circulation related to anthropogenic influences. Instead, they conclude that natural variations in the strength and speed of the thermohaline circulation can explain the observed behavior (for example, see here , here, and most recently, here)

The latest on these has just appeared in the scientific publication Geophysical Research Letters, authored by Tim Boyer and colleagues who hail primarily from the U S. National Oceanic and Atmospheric Administration (NOAA). Boyer et al. examined salinity trends in the waters at various depths and latitudes covering the North Atlantic Ocean. They found, overall, a general salinity increase over the entire basin. Of particular interest, as shown in Figure 1, is that while the waters of the tropical latitudes showed a fairly steady trend towards enhanced salinity since the beginning of their study period (1955) to the end (2006), in the northern latitudes, the waters showed a clear freshening trend lasting from about the late 1960s through the early-to-mid 1990s. Since then, however, the fresh water content of the northern latitudes of the Atlantic Ocean has been declining.


Figure 1. Equivalent freshwater content for different areas of the North Atlantic Ocean 0–2,000 meters (1955–1959) to (2002–2006). The trend in the high latitudes of the North Atlantic is indicated by the green line. (from Boyer et al., 2007)

This freshening trend of the high latitude Atlantic coupled with increasing salinity in the lower latitudes undoubtedly made it seem, at least when it was occurring, that the thermohline circulation was slowing down as a result of global warming—as reported by Curry et al. (2003) whose data ended in 1999. However, the freshening trend apparently ended in the mid-1990s and the increasing salinity in the north is an indication that the thermohaline circulation is still quite healthy, and in fact, that it has likely strengthened over the past 10 years or so—counter to the suggestions of Curry et al. (2003) and Bryden et al., (2005).

How many of you saw any headlines proclaiming this?

Interestingly, there may be a tie-in to Atlantic hurricane activity in all of this. For years, hurricane guru Bill Gray has been saying that the strength of the thermohaline circulation is an important determinant of Atlantic hurricane activity. And some folks have given Dr. Gray a lot of grief about this, basically claiming that he doesn’t know what he is talking about.

But these findings by Boyer et al. seem to give added credence to Dr. Gray’s idea. If one takes the behavior of the fresh water content of the northern portions of the North Atlantic to be an indicator of thermohaline strength (fresher is weaker, more salty is stronger), then there seems a strong association between circulation strength (Figure 1) and Atlantic hurricanes (Figure 2). Figure 2 shows the frequency of hurricanes (and major hurricanes) in the Atlantic basin. From the late-1950s through the early 1970s, when the high latitude, subpolar Atlantic waters were relatively salty, hurricane activity was rather high, as the subpolar Atlantic freshened from the early 1970s to the mid-1990s, Atlantic hurricane activity diminished. Hurricane activity once again picked up in intensity and frequency in 1995 at precisely the time when the subpolar waters began to increase in salinity.


Figure 2. Annual number of hurricanes and major hurricanes observed in the Atlantic basin, 1930-2006 (source: National Hurricane Center, www.nhc.noaa.gov).

And the timing of the salinity trends in the subpolar Atlantic meshes nicely with the timing of the oscillations of the Atlantic Multidecadal Oscillation (AMO), a natural pattern of Atlantic sea surface temperature fluctuations that has also been tied together with hurricane activity and thermohaline circulation strength. The oscillations in the AMO can be traced back many hundreds of years—a strong indication that it is indeed natural processes, rather than anthropogenic activities, that primarily drive these fluctuations

For those hoping for an end to the current period of enhanced Atlantic tropical cyclone activity, the results of Boyer et al. don’t offer much good news, for they don’t indicate that the current trend towards a saltier subpolar Atlantic is ending. The same can be said for those hoping for an “abrupt climate change” to occur to demonstrate that we have helped throw the climate out of whack. This seems to present a sort of quandary—strong thermohaline circulation, no abrupt climate change, but more active hurricane seasons, or weakening thermohaline circulation, possibility of abrupt climate change but weaker hurricanes. Which should a loyal alarmist hope for? Actually, this is not a quandary at all, but a win-win situation for them—for no matter what happens, they can blame it on fossil fuels, despite evidence to the contrary. What’s new.

References:

Boyer, T., et al., 2007. Changes in the freshwater content of the Atlantic Ocean, 1955-2006. Geophysical Research Letters, 34, L16603, doi:10.1029/2007GL030126.

Bryden, H.L., et al., 2005. Slowing of the Atlantic meridional overturning circulation at 25ºN. Nature, 438, 655-657.

Curry R., et al., 2003. A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 426, 826-829.




No Comments

No comments yet.

RSS feed for comments on this post.

Sorry, the comment form is closed at this time.

Powered by WordPress