Category 6â„¢

El Niño gone; La Niña on the way?

By: JeffMasters, 5:33 PM GMT on February 28, 2007

The El Niño event of 2006-2007 is over. Ocean temperatures in the Equatorial Eastern Pacific have cooled rapidly over the past four weeks, resulting in near-normal water temperatures and an end to the El Niño event that began in September 2006. By definition, an El Niño episode occurs when Sea Surface Temperatures (SSTs) are at least +0.5ºC above normal in the region 120°W-170°W and 5°S-5°N (called the Niño 3.4 region). SSTs in this region reached +0.5ºC above normal in September, and fell below +0.5ºC above normal in late January. A time series of the departure of SST from normal (Figure 1) shows the rapid cooling over the past four weeks to near-normal values in this El Niño 3.4 region (black box on the plots).

Figure 1. Departure of Sea Surface Temperatures from normal for the past four weeks. The black box marks the region 120°W-170°W and 5°S-5°N (called the Niño 3.4 region). Note the rapid cooling to below-normal values in portions of this box. Image credit: NOAA Climate Prediction Center.

What does this mean for Atlantic hurricane season?
The demise of El Niño is bad news for those living along the hurricane-prone areas of the Atlantic coast. El Niño conditions tend to suppress Atlantic hurricane activity, as occurred in the 2006 hurricane season. It is extremely unlikely we will see a return to El Niño conditions this fall. A decay of El Niño this time of year is very rarely followed by a resurgence later in the year, and only one of the 20 or so computer models used to forecast El Niño is forecasting this to happen this year. It is much more likely that we will see a full-fledged La Niña episode develop. Indeed, La Niña may be already be well on its way--NOAA chief Conrad Lautenbacher remarked in a press release today, "we're seeing a shift to the La Nina, it's clearly in the data". He was refering to a large pool of cooler than normal waters that has developed in recent weeks in the sub-surface waters of the Equatorial Eastern Pacific. This is a prime situation for a La Niña to develop, and several of the long-range computer models are predicting La Niña conditions for the coming hurricane season (Figure 2). These models are not very reliable, however, and it is equally probable that we will see El Niño-neutral conditions--the absence of either a La Niña or El Niño--for the coming hurricane season. La Niña conditions usually cause Atlantic hurricane seasons that are much more active than average, so El Niño-neutral conditions would probably be more welcome than a La Niña. Remember, though, that the worst hurricane season on record--the infamous Hurricane Season of 2005--occurred with El Niño-neutral conditions. I am expecting a much more active hurricane season than the mild season of 2006 as a result of this month's demise of El Niño.

Figure 2. Computer model forecasts of the departure of SST from normal in the region 120°W-170°W and 5°S-5°N (called the Niño 3.4 region). Temperatures +0.5ºC above normal in this region indicate an El Niño episode; temperatures -0.5ºC below normal indicate an La Niña. Three of the 14 models plotted predict La Niña conditions during the upcoming hurricane season (ASO, August-September-October), one model predicts El Niño conditions, and the other ten predict El Niño-neutral conditions. Image credit: International Research Institute ofr Climate and Society.

I'd like to welcome our new featured blogger, Mike Theiss! Mike is a professional weather photographer and storm chaser, and will be sharing his awesome storm photos with us for the coming tornado season. He also documents all landfalling U.S. hurricanes (check out his amazing Katrina videos), so tune in this hurricane season to his blog!

Jeff Masters

Oscar time for Al Gore's movie

By: JeffMasters, 4:10 PM GMT on February 26, 2007

If you haven't seen Al Gore's global warming movie, "An Inconvenient Truth", it's time you watched this important film. The movie took home the Oscar award for best documentary feature of 2006 at last night's Academy Awards, and also won an Oscar for best original song, singer Melissa Etheridge's "I Need to Wake Up." As I wrote in a movie review last year, Gore does a good job educating the non-scientist about the science of climate change and the dangers it poses. The only major flaw scientifically in the movie is the unwarranted connections he makes between climate change and severe weather events such as Hurricane Katrina and the record number of tornadoes in 2004. I gave his science a "B" overall. I thought the movie was a bit too long and was excessively political, but definitely worth seeing (2.5 stars out of four). It is difficult to make a scientifically accurate movie about climate change that will also be interesting enough to do well at the theaters; an "An Inconvenient Truth", while admittedly imperfect, does a respectable job educating us about climate change and the challenges and dangers it poses.

Should "An Inconvenient Truth" be shown in schools?
According to a recent blog posted at, "An Inconvenient Truth" has a become a required part of the science curriculum in some countries. One of the producers of the film, Laurie David, recently offered 50,000 free copies of the $19.99 DVD to National Science Teachers' Association (NSTA) for use in U.S. classrooms. The NSTA turned down the offer on the grounds that the NSTA has a 2001 policy against "product endorsement", and a fear that distributing the film would place "unnecessary risk upon the [NSTA] capital campaign, especially certain targeted supporters." points out that one of these targeted supporters is oil giant ExxonMobil, and questions whether concern about losing funding from ExxonMobil influenced the decision not to take the free movies. I don't have a problem with the NSTA rejecting the free movies on the grounds that Al Gore's presentation is politicized. However, as pointed out in the post, NSTA does not offer much content on climate change in their list of recommended materials. One of the recommended books, "Global Warming: Understanding the Debate", has no business being on their recommended reading list. This book is written by Kenneth Green, a fellow of the American Enterprise Institute (AEI). This fossil-fuel funded think tank recently offered $10,000 to scientists willing to criticize the recent landmark 2007 Summary of Policy Makers climate change report issued by the United Nations-sponsored Intergovernmental Panel on Climate Change (IPCC). AEI offered to award the money to scientists who would "thoughtfully explore the limitations of climate model outputs", as explained in an article in the UK Guardian. Given the lack of quality climate change education material it offers to teachers, NSTA needs to seriously rethink their recommended offerings on this important subject. If they are going to continue to recommend a book written by the fossil fuel industry-funded American Enterprise Institute, they should recommend Al Gore's movie as well. The two best books for teaching about climate change are missing from the NSTA's recommendations: Robert Henson's excellent Rough Guide to Climate Change (high school level) and The North Pole Was Here (grades 6-9), by New York Times climate change writer Andrew Revkin.

I'll be back Wednesday with a look at the weather of January 2007--the warmest January on record, globally.

Jeff Masters

Book and Movie Reviews Climate Change

Future abrupt loss of Arctic sea ice

By: JeffMasters, 5:53 PM GMT on February 22, 2007

We've heard a lot about the melting sea ice in the Arctic. The steady loss of the polar ice cap may endanger the polar bear, but provide new shipping channels and opportunities for commercial exploitation of the Arctic. Since 1979 (the year satellite imagery of the north pole first became available), the areal coverage of the Arctic sea ice has shrunk by about 10% in winter (4% per decade) and 20% in summer (8% per decade). The loss of sea ice, when plotted on a graph (Figure 1), has roughly followed a straight line over time. There are a few noisy ups and downs, reflecting colder and warmer years than average. A trend that approximately follows a straight line is called a "linear" trend. A continued linear summertime 8% per decade loss of sea ice would leave the summertime Arctic Ocean ice-free by 2100. The ocean would still partially freeze in winter, with about 50% of the ocean covered with ice.

Figure 1. Average September Arctic sea ice coverage as observed by satellites between 1979 and 2006. Image credit: NOAA's National Snow and Ice Data Center.

However, there is a distinct possibility that Arctic sea ice loss may show a sudden non-linear decline in coming years. The loss of sea ice with time may no longer follow a nice straight line, but instead suddenly accelerate, allowing the Arctic sea ice to suffer a sudden and complete disintegration in just a decade. The result would be an ice-free Arctic Ocean for the first time since before the last ice age. This possibility was explored in a December 2006 paper (Holland et al.), titled "Future abrupt reductions in the summer Arctic sea ice". The authors ran the Community Climate System Model, one of the top climate models used to formulate the "official word" on climate, the 2007 Intergovernmental Panel on Climate Change (IPCC) report. The model was run for the years 1979-2006, and successfully predicted the 20% loss of summer sea ice during that period. The model then assumed that levels of greenhouse gases would continue to increase, until a doubling of CO2 levels occurred in 2100. This is considered a "middle-of-the-road" scenario, and assumes a reasonable sequence of events will unfold over the coming decades: humans will make some modest efforts to control greenhouse emissions, but not enough to prevent dangerous climate change. The model found that Arctic sea ice continued to decline linearly until about 2024, resulting in about 60% sea ice coverage in September (Figure 2). During this period, the vertical thickness of the sea ice declined from about four meters to one meter. Beginning in 2025, the rate of sea ice loss suddenly tripled, resulting in the total loss of the summertime polar sea ice by 2040. The authors theorize that once the ice reaches a critical thickness--in this case, one meter--the processes that create open water suddenly become more efficient, resulting in a rapid disintegration of the remaining ice.

Figure 2. September Arctic sea ice extent observed in 1979 (yellow line), and 2005 (white area). The predicted coverage of sea ice by Holland et al. (2006) is shown for 2015 (red line) and 2040 (green line). Their model predicts that sea ice in summer by 2040 will occur only in narrow bands along the Canadian Arctic coast. However, there will still be about 50% sea ice coverage in winter. Original image taken from NASA.

The authors tested 11 other models that were also used to formulate the 2007 IPCC report. Six of these 11 models also showed similar sudden losses of the summer sea ice. When these models were run assuming that dramatic efforts to cut greenhouse gas emissions will be made over the coming decades, only 3 of the 15 models tested showed sudden summer sea ice losses.

The authors concluded that "abrupt changes in the summer Arctic sea ice cover are quite likely and can occur early in the 21st century, with the earliest event in approximately 2015". Given that just over 50% of the models tested show such an effect, it is by no means a sure thing that we'll see a total loss of Arctic sea ice by the middle of the century. However, the results should be impetus to drastically cut greenhouse emissions soon, as the probability of an ice-free Arctic increases significantly if we do nothing.

This is the fourth in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday. Part five is: Why should we be concerned about an ice-free Arctic Ocean? This one might wait a few extra days, as there are other topics I may want to talk about. My next blog will be Monday.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

Holland, M.M., C.M. Bitz, B. Tremblay, 2006 "Future abrupt reductions in the summer Arctic sea ice", Geophysical Research Letters, 33, L23503, December 2006.

Climate Change

Why Arctic sea ice is shrinking

By: JeffMasters, 10:25 PM GMT on February 20, 2007

Second of three vacation blogs from Dr. Masters. Enjoy.

Since 1979, coverage of Arctic sea ice has shrunk by about 10% in winter and 20% in summer. The vertical thickness of the ice has also shrunk. According to the "official" Intergovernmental Panel on Climate Change (IPCC) report in 2001, Arctic sea ice decreased in thickness by about 40% between 1958 and 1997. This figure came from submarine sonar measurements taken over five cruises in the autumns of 1958, 1960, 1962, 1970, and 1976 when compared with three cruises in the autumns of 1993, 1996, and 1997 (Rothrock et. al, 1999). However, according to modeling studies by Holloway and Sou (2001), these results are highly uncertain, since not all years or locations in the Arctic were sampled. They found that if the first five cruises had been done just one year earlier (September 1957, 1959, 1961, 1969, 1975) and the three latter cruises had been done one year later (September 1994, 1997, 1998), the sonar measurements would have shown only an 11%-15% decrease in thickness. Arctic sea ice varies greatly in thickness, and currents and winds are always pushing the ice around, making it difficult to measure how the average thickness has been declining.

Warmer air and water temperatures have contributed to the sea ice decline

Annual average surface temperature has increased about 1 degree C since 1980 over the Arctic, which accounts for much of the sea ice melt. In addition, some melting has occurred from beneath the ice, due to warmer ocean waters. Global warming has heated up both the North Pacific and North Atlantic waters significantly over the past 30 years. Warmer waters have been brought into the Arctic Ocean from the Pacific via an ocean current flowing through the Bering Strait between Alaska and Russia, and from the Atlantic via an ocean current flowing northwards along the European coast.

Figure 1. The Arctic Oscillation (AO) index from 1899 - 2006. The AO is a measure of the difference in surface pressure between the north pole and about 45 degrees north latitude. Image credit: Dave Thompson of Colorado State University.

Wind patterns are a major cause of sea ice loss

The Arctic Oscillation is an observed natural pattern of surface pressure variations in the Northern Hemisphere. The "positive index" of the AO is defined when the surface pressure is below normal at the north pole and above normal at about 45 degrees north latitude. Positive Arctic Oscillation conditions steer storms farther north, bringing stronger surface westerly winds in the North Atlantic and warmer and wetter than normal conditions to the Arctic and northern Europe. The winds and ocean currents during the positive Arctic Oscillation mode tend to drive sea ice from west to east along the north shore of Canada, then out of the Arctic Ocean through the channel of water to the east of Greenland (Fram Strait).

When one looks at the wintertime pattern of the Arctic Oscillation (AO) over the past 100 years, a mostly random pattern of positive and negative AO modes is apparent (Figure 1). However, one anomalous period is very striking: a string of seven consecutive years with a positive AO, including two years (1989 and 1990) with the highest AO index ever observed. During this period, strong westerly winds rapidly flushed more than 80% of the oldest, thickest sea ice out of the Arctic Ocean, leaving most of the Arctic covered with ice less than three years old (Figure 2). Younger ice is much thinner, and melts much more readily. Rigor and Wallace (2004) estimate that at least half of the loss of sea ice in the Arctic since 1979 is due to these six years of strange weather with very low surface pressure over the Arctic. Did climate change cause this unusual pattern between 1989 and 1995? It is possible, but no one has published any papers showing how this might have occurred. For now, the assumption is that this major loss of Arctic sea ice due to wind patterns between 1989-1995 is natural.

The big concern is that since the strange positive Arctic Oscillation years of 1989-1995, a number of years with negative AO have occurred. Normally, during negative AO years, ice extent and thickness increase in the Arctic. But instead, ice extent and thickness during 2002-2006 have shown an unprecedented series of record minima, giving rise to fears that we are on our way to an ice-free Arctic later this century.

Figure 2. The change in age and thickness of sea ice between 1987 and 2005. In 1987, most of the Arctic sea ice was old and thick, generally more than ten years old. A period of strong positive Arctic Oscillation conditions between 1989 and 1995 created winds and currents that flushed most of this old ice out of the Arctic Ocean, through Fram Strait to the east of Greenland. The new ice that replaced the old ice is much thinner. Image credit: Rigor, I. G., and J. M. Wallace (2004), "Variations in the age of Arctic sea-ice and summer sea-ice extent," Geophys. Res. Lett., 31, L09401, doi:10.1029/2004GL019492.

This is the third in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday over the next two weeks. Next blog: Future abrupt loss of Arctic sea ice.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

Jeff Masters

Holloway, G. and T. Sou, 2001, "Has Arctic Sea Ice Rapidly Thinned?", Journal of Climate 15, p1691-1701, 2001.

Rigor, I. G., and J. M. Wallace (2004), "Variations in the age of Arctic sea-ice and summer sea-ice extent," Geophys. Res. Lett., 31, L09401, doi:10.1029/2004GL019492.

Rothrock D.A., Y. Yu, and G.A. Maykut, 1999: "Thinning of the Arctic sea ice cover." Geophys. Res. Lett., 26, 3469-3472.

Climate Change

The pole star and Arctic climate change

By: JeffMasters, 5:01 PM GMT on February 15, 2007

Hello Weather Underground bloggers, Aaron here. Dr. Masters is on vacation so I'll be posting a series of vacation blogs for him. This is the first.

In my previous blog, I mentioned how the Arctic Climate Impact Assessment (ACIA), a 2004 study of Arctic climate compiled by 300 scientists over three years, found that the recent increase in Arctic temperatures was probably due to human-emitted greenhouse gases. Greenhouse skeptics attacked the ACIA report and its conclusion, pointing out that the Arctic was much warmer than today during the period 4,000 - 7,000 years ago. For example, Dr. Patrick Michaels said in a 2004 interview with CNS news, a conservative Internet news service:"It was warmer 4 to 7,000 years ago [in the Arctic.] Every climatologist knows that. I saw no mention of that in the Arctic report," Michaels said. He added that the past warming of the Arctic couldn't possibly be blamed on greenhouse gas emissions since it occurred long before the industrial era.It is true that Arctic summers were warmer during the period 4,000 - 7,000 years ago. The mean July temperature along the northern coastline of Russia may have been 2.5 to 7.0 �C warmer than present, and Scandinavian summer temperatures were 1.5 to 2 �C higher than at present. The warming was caused by changes in the amount of sunlight the north pole gets in summer due to variations in the Earth's orbit.
Figure 1. Summertime temperatures in the Arctic during the Mid-Holocene Warm Period (about 6,000 years ago), compared to today's temperatures. Image credit: NOAA.

Earth's orbital variations
Earth's orbit is not perfectly circular, which means that we are closest to the sun in December and farthest in July. Thus, the Southern Hemisphere gets more sunlight in their summer (December) than the Northern Hemisphere does in their summer (July). The Earth's current 23.5 degree tilt keeps the north pole pointed towards the star Polaris, which results in the north pole being pointed at the sun in December (Northern Hemisphere winter). The Earth slowly wobbles around its axis with a period of 23,000 years a href=(precession) so that the pole star gradually changes with time. During the period 4,000 - 7,000 years ago, the pole star was Thuban in the constellation Draco, and the north pole was pointed at the sun during Northern Hemisphere summer. This resulted in much warmer summer temperatures in the Arctic, since there was more summer sunlight. Conversely, winter temperatures were colder, since the Earth was farther from the sun during the Northern Hemisphere winter. The Arctic will again be much warmer in summer 16,000 to 19,000 years from now when the cycle repeats and Thuban is once more the pole star. The reason the ACIA failed to mention this climate period is that the average temperature in the Arctic remained about the same during the period 4,000 - 7,000 years ago; it was just the summers that warmed. The warming since 1980 and the 1930s were warmings over all seasons, so it was misleading for the climate skeptics to compare Arctic temperatures 4,000 - 7,000 years ago with these modern warmings.

This is the second in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday. Next blog: Why the Arctic sea ice is shrinking.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

Jeff Masters

Climate Change

Arctic climate change: the past 100 years

By: JeffMasters, 1:56 AM GMT on February 12, 2007

The Arctic is a region particularly sensitive to climate change, since temperatures are, on average, near the freezing point of water. Slight shifts in the average temperature can greatly change the amount of ice and snow cover in the region, due to feedback processes. For example, as sea ice melts in response to rising temperatures, more of the dark ocean is exposed, allowing it to absorb more of the sun's energy. This further increases air temperatures, ocean temperatures, and ice melt in a process know as the "ice-albedo feedback" (albedo means how much sunlight a surface reflects). The 20% loss in Arctic sea ice in summer since 1979 has given rise to concerns that this "ice-albedo feedback" has taken hold and will amplify until the Arctic Ocean is entirely ice-free later this century. Should we be concerned? Has the Arctic been this warm in the past and the sea ice survived? The answers are yes, and yes.

Figure 1. Annual average change in near surface air temperature from stations on land relative to the average for 1961-1990, for the region from 60 to 90° north. Image credit: The Arctic Climate Impacts Assessment (ACIA).

The past 100 years
The Arctic Climate Impact Assessment (ACIA), published in November 2004, was a uniquely detailed study of Arctic climate compiled by 300 scientists over three years. The study found that while temperatures in the Arctic have increased significantly since 1980 (Figure 1), there was also a period in the 1930s and 1940s when temperatures were almost as warm. If one defines the Arctic as lying poleward of 62.5° north latitude (Polyakov, 2003), the 1930s and 1940s show up being warmest period in the past 100 years. Looking at Figure 1, one cannot dismiss the possibility that temperatures in the Arctic oscillate in a 50-year period, and we are due for a cooling trend that will take temperatures below normal by 2030.

However, the period since 1980 was a time when the entire globe (except the bulk of Antarctica) warmed, and the 1930s and 1940s were not. Thus, the 1930s and 1940s warming in the Arctic is thought to be fundamentally different. Furthermore, the past 20 consecutive years have all been above normal in temperature, whereas during the 1930s and 1940s there were a few cooler than average years interspersed with the very warm years. A detailed breakdown by month and region of the 100-year history of Arctic temperatures was performed by Overland et al. (2004). They found no evidence of a 50-year cycle in Arctic temperatures, and concluded that the warming since 1980 was unique. However, they stopped short of blaming the recent warming on human-emitted greenhouse gases (anthropogenic forcing). The ACIA, though, concluded that humans were likely to blame for the recent Arctic warming, but not definitely:

It is suggested strongly that whereas the earlier warming was natural internal climate-system variability, the recent surface air temperature changes are a response to anthropogenic forcing. There is still need for further study before it can be firmly concluded that the increase in Arctic temperatures over the past century and/or past few decades is due to anthropogenic forcing."

This is the first in a series of five blogs on climate change in the Arctic that will appear every Monday and Thursday over the next two weeks. Next blog: The skeptics attack the ACIA report--and how the position of the pole star is indicative of Arctic climate change.

Also, be sure to visit our new Climate Change blog, written by Dr. Ricky Rood of the University of Michigan.

Jeff Masters


Overland, J.E, M.C. Spillane, D.B. Percival, M. Wang, H.O. Mofjeld (2004), "Seasonal and Regional Variation of Pan-Arctic Surface Air Temperature over the Instrumental Record", Journal of Climate, 17:17, pp3263-3282, September 2004.

Polyakov, V., et al. (2003), "Variability and Trends of Air Temperature and Pressure in the Maritime Arctic, 1875-2000", Journal of Climate, 16, 2067-2077.

Climate Change

Hurricane research flights grounded

By: JeffMasters, 2:24 PM GMT on February 08, 2007

For the first time since NOAA began flying research aircraft into hurricane in the 1950's, there is no money to fund airborne hurricane research for an upcoming hurricane season. NOAA's state-of-the-art flying weather research laboratories, the two P-3 Orion hurricane hunter aircraft, may sit idle this hurricane season due to a lack of funding. NOAA's Hurricane Research Division (HRD) usually receives several million dollars each year to perform hurricane research using the P-3's. However, funding for HRD has steadily declined over the past decade, forcing HRD to reduce staff and cut back on hurricane research. Now, this key form of hurricane research has been zeroed out by NOAA. It is possible that the National Science Foundation will step in and fund one P-3 research project, though--there is interest in taking real-time P-3 Doppler radar data and putting it into one of NOAA's experimental hurricane research computer models (the HWRF model). It is also possible that if the President's newly-proposed budget gets approved (which contains an extra $2 million in funding for hurricane intensity research), some of that money will go towards keeping the P-3s flying. The National Hurricane Center (NHC) does have money to keep the P-3s flying this year, but not for reasearch projects. Flights done for NHC would be strictly operational--one altitude, one airplane at a time, with the intent of providing center fixes and surface winds estimates. HRD scientists would be allowed to take research data, but would not be able to fly both P-3s at once, or do custom flight patterns to use the P-3s' Doppler radar and other advanced instrumentation to gather state-of-the-art research data. No follow-up work on last years promising field study that examined the effect of African dust on suppressing hurricane activity will be performed. And with the Air Force C-130 hurricane hunters receiving the advanced SFMR surface wind measuring instrument this year, it is questionable how much flying time the P-3s will get from NHC.

NOAA's P-3 hurricane hunter research aircraft. Image credit: NOAA/AOC.

With zero money allocated to fund one of the most important types of hurricane research, one has to wonder--what are NOAA and Congress thinking? While improvements in computer models, better satellite data over oceanic regions, and better forecasting techniques are primarily responsible for the 43% improvement in hurricane track forecasts in the past 15 years, research flights performed by the P-3s are also a big reason. For example, the now routine flights by the NOAA high-altitude jet to sample the large-scale environment around a hurricane improves tracks forecasts by perhaps 20% on its own, when it flies. This advancement grew out of a multi-year research project conducted by the P-3s in the 1980s and 1990s. Continued hurricane research by aircraft is essential if we are to continue improving track forecasts, and do a better job at forecasting intensity--which has only improved 17% in the past 15 years. The National Science Board, in a report issued September 29, 2006, called for an increase of $300 million per year in hurricane research funding. The National Hurricane Research Initiative Act, was introduced in the Senate in September to fully fund the National Science Board's recommendations. I presented a long report on these initiatives in a blog in October.

Given the huge return on our investment the NOAA P-3s have already paid, and the critical need to improve our understanding of hurricanes, it is imperative that we not let NOAA's hurricane research aircraft sit idle. It's like signing Roger Clemens to get you to the World Series, then deciding to use him just to pitch batting practice. I urge you to write your Representatives to approve more funding for hurricane research, and ask your Senators to support S. 4005, the National Hurricane Research Initiative Act of 2006. The act is co-sponsored by all four of Louisiana and Florida's senators. When the bill comes before committee or the full Senate, I will be sure to post a follow-up blog urging you to write your Senators again.

My next blog will be Monday.

Jeff Masters

Florida's tornadoes and El Niño

By: JeffMasters, 4:54 PM GMT on February 05, 2007

Friday's devastating tornadoes in Central Florida, where 20 people lost their lives, marked the second most deadly tornado outbreak in Florida history. Only the February 22-23, 1998 tornado outbreak in Kissimmee, which killed 42 and injured 260, was worse. Two of Friday's tornadoes were rated as EF-3 (Category 3 on the new Enhanced Fujita Scale), with winds near 160 mph. The Enhanced Fujita Scale became the official scale for rating tornadoes as of February 1, 2007, and Friday's tornadoes were the first to be ranked with the new damage scale. The new scale replaces the old Fujita Scale, which required winds of a tornado to be much higher in order to get an F3 or higher rating. Modern engineering studies have determined that devastating damage can occur at much lower wind speeds, and that the Fujita Scale did a poor job of correlating between damage and wind speed. For example, tornadoes capable of causing incredible damage (EF 5 rating) are now known to occur at wind speeds of 200 mph and higher. On the old F-Scale, an F-5 rating started at 261 mph. See the Tornado FAQ for a full comparison of the old Fujita Scale with the new Enhanced Fujita Scale.

Strong and violent tornadoes are rare in Florida, but when they do occur, it tends to be during winter when a moderate or strong El Niño event is occurring. More than 60% of Florida's killer tornadoes occur between midnight and noon, which was also the case with Friday's outbreak. This was the second major Florida tornado outbreak this winter. Earlier this winter, a series of three tornadoes, including two rated at F2, hit near Daytona Beach on Christmas Day, injuring 16 people. During El Niño winters, the jet stream winds tend to be stronger over Florida, a key ingredient needed for tornado activity (Figure 1).

Figure 1. Incidence of strong (F2 and higher) tornadoes over Florida between 1950 and 1998. Note the highest numbers of strong tornadoes occurred during the major El Niño years of 1983 and 1998. Image credit: NOAA.

Jeff Masters

Groundhogs, tornadoes, hurricanes, and the new IPCC report

By: JeffMasters, 5:44 PM GMT on February 02, 2007

Punxsutawney Phil, the fearless groundhog forecaster of Punxsutawney, Pennsylvania, did not see his shadow this morning, signifying that winter will come to an early close. Phil can expect this to be a common occurrence in the coming millennia, according to the newly released Summary of Policy Makers issued today by the United Nations sponsored Intergovernmental Panel on Climate Change (IPCC). In their fourth report since 1990, the IPCC offers its strongest language yet that Earth's climate is warming and humans are largely responsible:

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level."

"Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes, and wind patterns."

Earth has warmed, sea levels have begun to rise at an accelerated rate, and Northern Hemisphere snow cover has decreased substantially over the past 150 years (Figure 1). These facts are not controversial. The big change from the IPCC's last report, in 2001, is the level of confidence on if humans are to blame. In that report, human-emitted (anthropogenic) greenhouse gases were estimated to be likely responsible for Earth's temperature increase (67-90% chance), while the new report says it is very likely (greater than 90% chance).

Figure 1. Observed changes in (a) global average surface temperature; (b) global average sea level rise from tide gauge (blue) and satellite (red) data and (c) Northern Hemisphere snow cover for March-April. All changes are relative to corresponding averages for the period 1961-1990. Smoothed curves represent decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals estimated from a comprehensive analysis of known uncertainties (a and b) and from the time series (c). Image credit: FIGURE SPM-3 from the Summary of Policy Makers from the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report.

Predicted temperature rise
The 2007 IPCC report predicts temperature rises of 1.1 - 6.4°C (2 - 11.5°F) by 2100. This is a wider range than the 1.4 - 5.8°C increase given in the 2001 report. However, the 2007 report goes on to say that their best estimate for temperature rise is 1.8 - 4°C (3.2 - 7.1°F).

Predicted sea level rise
The 2007 report predicts that sea level rise by 2100 will be .6 - 1.9 feet (18 - 58 cm). An additional 3.9 to 7.8 inches (10 to 20 cm) are possible if the recent surprising melting of polar ice sheets continues. The 2001 IPCC report gave a much wider range for sea level rise: .3 - 2.9 feet (8 - 88 cm).

What does the IPCC say about stronger hurricanes?
The IPCC did a good job with their treatment of how global warming is affecting hurricanes. Here's their carefully worded statement on the observed changes:

There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

Later in the report, there is a table that shows that there has likely (>66% chance) been an increase in strong hurricanes since 1970 in some regions. It isn't mentioned, but the Atlantic is the region where this increase has been most notable. Also in that table is the assertion that it is more likely than not (>50% chance) that there has been a human contribution to this trend. This statement was leaked to the press yesterday, and resulted in speculation that the IPCC concluded that stronger hurricanes like Katrina were due to human-caused global warming. However, there is a footnote on the table, which wasn't part of yesterday's leaked press reports: "Magnitude of anthropogenic contributions not assessed. Attribution for these phenomena based on expert judgment rather than formal attribution studies."

In other words, the link between stronger hurricanes and global warming is a theory (expert judgment) and is not a conclusion of the IPCC. It is reasonable to theorize that some human contribution is responsible for the increase in strong hurricanes in the Atlantic since 1970, since this increase does correlate so well with the observed increase in sea surface temperatures. However, as mentioned in the earlier paragraph, it is difficult to make a strong statement saying that global warming is responsible for stronger hurricanes, due to the high natural variability of these storms and the poor observational record: "multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity." The IPCC table is confusing, and I believe it was a mistake to assign a probability of how likely a human contribution to hurricane intensity has been. There is not enough good science to make a sound judgment, and this section of the table should have been left blank.

Finally, the IPCC projection for how climate change will affect hurricanes in the future is pretty non-controversial, since they don't attach any numbers saying how large these effects will be:

Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.

Overall, the IPCC statements on hurricanes are very similar to those adopted by the World Meteorological Organization (WMO) in December, as I discussed in an earlier blog. The WMO report concluded, "Though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point."

What does the IPCC say about stronger tornadoes?
In the wake of today's devastating tornadoes that swept through Central Florida, killing at least 19 people, it is important to ask how climate change might be affecting tornadoes. Well, we don't have good enough long-term observational records of tornadoes to tell, and climate models don't shed any light on the issue, either. Here's the relevant statement in the 2007 IPCC report:

There is insufficient evidence to determine whether trends exist in small scale phenomena such as tornadoes, hail, lighting, and dust storms.

I'll have much more on the IPCC report next week, as will Dr. Ricky Rood of the University of Michigan, who will be writing in our new featured Climate Change blog. Who knows, if we want to collect a quick $10,000, we can "thoughtfully explore the limitations of climate model outputs", as explained in an article posted by the UK Guardian today.

Jeff Masters

Climate Change

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Category 6â„¢


Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather