Forecasting “Tipping Points” in the Climate System

The term “tipping point” has been used to describe a change in the environment such that a different environment regime results. This term has been applied most recently with respect to the Arctic sea ice coverage and climate (e.g., see http://www.physorg.com/news6558.html). A tipping point that results in serious, negative impacts on societal and environmental conditions could be catastrophic.

The paper by Peters, D.P.C., R.A. Pielke Sr., B.T. Bestelmeyer, C.D. Allen, S. Munson-McGee, and K.M. Havstad, 2004: Cross-scale interactions, nonlinearities, and forecasting catastrophic events. Proceedings of the National Academy of Sciences, 101, No. 42, 15130-15135, concludes that,

“Catastrophic events share characteristic nonlinear behaviors that are often generated by cross-scale interactions and feedbacks among system elements. These events result in surprises that cannot easily be predicted based on information obtained at a single scale…..We show that decisions that minimize the likelihood of catastrophic events must be based on cross-scale interactions, and such decisions will often be counterintuitive. Given the continuing challenges associated with global change, approaches that cross disciplinary boundaries to include interactions and feedbacks at multiple scales are needed to increase our ability to predict catastrophic events and develop strategies for minimizing their occurrence and impacts.”

The media reports present forecasts that the Arctic sea ice will melt in the coming decades (e.g., see “Arctic ice could disappear in 55 years”). This forecast, and other multi-decadal climate change forecasts, are generally based on focusing on a single scale and type of climate forcing (the radiative effect of the added well-mixed greenhouse gases) and climate assessment tool (the numerical global climate models). However, reality likely will show a much more complicated behavior (e.g., see Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38).

Even with Arctic sea ice, as one example of a missing influence, the role of the deposition of black carbon on the sea ice has been ignored in these media discussions. As discussed in our August 29th blog, which is reproduced from the 2005 National Research Council report,

“Deposition of BC aerosols over snow-covered areas can result in changes to the surface albedo (Chylek et al. 1983). Further reductions in albedo occur due to the enhanced melting that accompanies the heating of absorbing soot particles in snow. Chylek et al. (1983) estimate this enhancement to be up to a factor of ten in the rate of melting. Recent model results indicate radiative forcings of +0.3 W m−2 in the Northern Hemisphere associated with albedo effects of soot on snow and ice (Hansen and Nazarenko 2004).”

Thus before the media and some scientists present what they claim are definitive climate forecasts of reaching a “tipping point”, and whether or not the climate system will have a catastrophic event, they should also present and critically assess cross-scale interactions and feedbacks among system elements which could result in a very different climate response from what they expect.

What About Antarctic Sea Ice Trends?

There is considerable media attention on Arctic sea ice trends, as we have discussed in our last few weblogs. The Independent highlights their latest news release as “Sea ice melts to record low because of global warming”. If this news article were accurate, and it is global warming (i.e., a global scale warming) that is causing the well below average summer ice cover in the Arctic, as documented at http://nsidc.org/data/smmr_ssmi_ancillary/regions/total_arctic.html#nsidc, we would also expect to see reductions of the Antarctic sea ice coverage (a large portion of which melts in the summer and then refreezes in the winter). For the period 1973-2002 we examined this issue and found no trend over time in Antarctic sea ice coverage (see Figures 8 and 9a in Pielke et al. 2004). A NASA press release from August 2002 reported on “Satellites show overall increases in Antarctic sea ice cover.”

Trend analyses to near the current time of Antarctic sea ice area and extent anomalies are available from The National Snow and Ice Data Center’s website . In their data, a continued increase in the areal coverage of Antarctic sea ice is evident!

Why is this not also reported by the media? In terms of a global warming effect, is the area reduction in the Arctic sea ice compensated by an area increase of sea ice in Antarctica?

The positive trend in the Antarctic, in contrast to the Arctic, raises questions about the causes of the sea ice trend. If it were a “global warming” signal (i.e., “global in extent”), we should expect similar behavior in both hemispheres. However, if warming is the reason for the reduction the area coverage of Arctic sea ice, it is a regional warming effect. As we have emphasized in our weblog, it is on the regional scale that we need to focus our attention with respect to human-caused and natural variations and long-term change of climate. The differences in trends between the Arctic and Antarctic emphasizes that we need a regional focus.

More on Arctic Sea Ice

The news media releases on Arctic sea ice continue, with caveats buried deep in the articles, if at all. An example of the latest release is from the BBC on September 28th entitled “Arctic ice ‘disappearing fast’.” No mention is made of the need to reconcile the National Snow and Ice Data Center (NSIDC) Arctic sea ice information with that of the University of Illinois data (see http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.jpg for their analysis of long-term trends). The University of Illinois shows the largest negative deviation from the long-term average in 1995. There is no question that there is a long-term decline in areal coverage (since 1973, as we have also documented in Pielke et al. 2003 , which is the time period we have reasonably robust areal coverage data from satellites), and that human climate forcings of all types must be playing some role, however, its temporal trends are more complicated than discussed in the BBC and other media articles.

As the BBC article summarizes,

“If the current trend can be ascribed in part to human-induced climate change, Mark Serreze sees major reasons for concern.

‘What we’re seeing is a process in which we start to lose ice cover during the summer,’ he said, ‘so areas which formerly had ice are now open water, which is dark.’

‘These dark areas absorb a lot of the Sun’s energy, much more than the ice; and what happens then is that the oceans start to warm up, and it becomes very difficult for ice to form during the following autumn and winter.’

‘It looks like this is exactly what we’re seeing – a positive feedback effect, a ‘tipping-point’.”

The idea behind tipping-points is that at some stage the rate of global warming would accelerate, as rising temperatures break down natural restraints or trigger environmental changes which release further amounts of greenhouse gases.

Possible tipping-points include:
• the disappearance of sea ice leading to greater absorption of solar radiation
• a switch from forests being net absorbers of carbon dioxide to net producers
• melting permafrost, releasing trapped methane.

This study is the latest to indicate that such positive feedback mechanisms may be in operation, though definitive proof of their influence on the Earth’s climatic future remains elusive.

We have examined the consequences of the removal of Arctic sea ice on summer climate in the Arctic in “Earth System Modeling- An Integrated Assessment Tool for Environmental Studies”. Glen Liston and Joseph McFadden of our research group ran the Regional Atmospheric Modeling System (RAMS) for June 1995 with assimilated observed sea ice and with the ice removed. What we found was that near-surface air temperatures are actually warmer over the Arctic Ocean (by over 1 degree Celsius) when the sea ice is present! The reason for the warming is that sea ice absorbs solar radiation and transfers some of this energy as sensible heat back into the atmosphere. Without the sea ice, while the ocean gains heat, the atmosphere is cooler. The difference in surface air temperature, with and without Arctic sea ice, is communicated to the atmosphere which could then alter tropospheric circulation patterns. Until the ocean warms enough through the addition of heat that is not present with the sea ice cover, the summer air temperature warming would be muted. This negative summer feedback is one example of the complexity of the Arctic climate with respect to both natural climate variability and human-caused climate change which has been ignored in the media releases.

This coming winter will provide an effective additional scientific test of whether the Arctic sea ice will continue to decline in areal coverage. A return to the larger winter coverage of earlier years will bring into question the statement of the BBC that “Arctic ice ‘disappearing fast’.” If the winter coverage repeats last year’s record low, however, than there will be support for the conclusions provided in the news release and of the NSIDC conclusions. Fortunately, in the case of Arctic sea ice coverage, we have a testable hypothesis in a reasonably short period of time. We will revisit this issue this winter. Meanwhile, we look forward to NSIDC and the University of Illinois reconciling their data, so a true scientific consensus can be achieved on this subject. Even the BBC caveats their conclusions,

“This study is the latest to indicate that such positive feedback mechanisms may be in operation, though definitive proof of their influence on the Earth’s climatic future remains elusive.”

The behavior of Arctic sea ice this winter and over the next few years provides data to more definitively address these issues.

Media Cherrypicking of the Areal Coverage of Arctic Sea Ice

There was a news report on September 16, 2005 in the online edition of “The Independent” which highlighted that “Global warming (is) ‘past the point of no return” (payment required to view the entire article). The article used sea ice coverage to make their point, and referred to Mark Serreze as the basis for their claim. However, the article was erroneous and illustrates the clear intent to bias a news release to support a particular perspective.

With Mark Serreze’s approval, I have copied below his response, dated September 19, 2005, with respect to The Independent news article. This response was sent to a mail group that questioned the article (http://groups.yahoo.com/group/climatesceptics).

“Folks:

I need to bring your attention to several key points regarding the article. Mr. Connor, who wrote the Article in the “Independent” has jumped the gun. My quotes stemmed from interviews back in mid-late August. They arose from an EOS article that I co-authored with Jonathan Overpeck and others. Mr. Connor’s article indicates that there will be a press release on September 20. There is no such release planned for that date. Apparently, he misconstrued statements from one of my colleagues. We are assembling a series of “talking points” regarding 2005 sea ice conditions, but this will only be released when all the facts are in.

John: According to our calculations, sea ice was still declining as of last Friday (see http://www.nsidc.org/data/seaice_index/). This is based on AMSR data. I think the Univ. IL information is based on SSM/I, but I’m not sure.

Will 2005 be a record? I don’t know. I know it will be close one way or the other. We’ll know in a couple of weeks. And as part of this investigation, we need to address discrepancies with the Univ. IL data.

Mr. Connor seems to have taken a wild guess that we will have a new record minimum. Maybe he will be right. The numbers that he quotes were apparently taken from the web site listed above, but they are based on incomplete information. We are tracking sea ice conditions closely, but as stated, we don’t yet know how 2005 will stand in comparison to other years. If it is a record, we will certainly let this be known.

In conclusion, Mr. Connor has “jumped the gun.” I am firmly convinced that at least part of what we are seeing in the Arctic is due to human influences. However, sensationalist articles like Mr. Connor’s only serve to further polarize what is already a very polarized issue. As I have reported in a number of peer reviewed articles, climate change is a complicated issue. As my colleague Dr. Polyakov has frequently pointed out, the Arctic is home to large natural fluctuations in climate.

I feel “ambushed” by Mr. Connor’s article.

I will make no further responses on this issue until the final numbers are in. And I will certainly not be talking with Mr. Connor.

Cordially,

Mark C. Serreze”

To clarify the actual current status of Arctic sea ice areal coverage, one source, can be found at the University of Illinois, including the Northern Hemisphere anomaly analysis as we have discussed in previous blogs. A second source of Arctic (and Antarctic) sea ice coverage is available from the National Snow and Ice Data Center.

The NSIDC website shows a more-or-less steady decline in the August sea ice coverage, with 2005 being the lowest in the period of record (1979-present). September data is not yet posted on that website, but according to Mark Serreze, September sea ice coverage continued to decline at least through the Friday before his e-mail. The largest contraction of the August sea ice is in the eastern Hemisphere (http://www.nsidc.org/data/seaice_index/n_extn.html).

However, the University of Illinois presents a somewhat different analysis. The current distribution of Northern Hemisphere Arctic sea ice (and snow cover), as viewed looking down on the North Pole, is presented in http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/arctic.shade.jpg, with the anomalies plotted in http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.365.jpg. The very low coverage in July and August is evident, but the melt for the remainder of the period was generally slower than average, such that at the current time the areal coverage is only slightly below the 1979-2000 average.

There clearly is a need to reconcile these two sets of analyses. However, it is clear that the news article was another example of media cherrypicking in order to advocate a particular perspective on climate change. More appropriately, we need to recognize that the assessment of Arctic sea ice coverage, and its variability and change over time, as it is affected by human-caused and natural climate forcings and feedbacks, is a complex scientific issue. The diversity of perspectives on these issues needs to be accurately presented in the media, which clearly was not the case with The Independent news article.

Is Global Warming Spatially Complex?

The short answer is Yes.

As discussed in Heat storage within the Earth system, the appropriate climate metric to assess global warming is ocean heat content in Joules. As was shown in that 2003 paper, the radiative imbalance of the climate system can be effectively assessed by monitoring changes in Joules of the ocean heat content over time, as the other stores of heat in the climate system are small. For example, in that paper, between the mid-1950s and the mid-1990s, a global radiative imbalance of + 0.3 Watts per meter squared was diagnosed, with half of this heating (+0.15 Watts per meter squared) above 300 m and the remainder between 300 m and 3 km. Since that study, the analysis of Willis et al. 2004 provides more recent ocean heat storage changes. As we diagnosed from their data (see Pielke and Christy), the radiative imbalance for the period 1993- mid 2003 was about 0.62 Watts per meter squared.

However, these estimates are based on a global ocean average heat storage change. The actual spatial trends in ocean heat content are actually quite complex (i.e., see Figure 4 in Willis et al. 2004). They found that most of the heating was in the southern hemisphere mid-latitude ocean down to a depth of 750 m. The sea surface temperature (SST) anomalies mirror this spatial complexity at the surface of the oceans (see http://www.osdpd.noaa.gov/PSB/EPS/SST/climo.html for a current map of the anomalies). On September 24, 2005 large areas of cool SST anomalies are evident in the southern hemisphere oceans, while large areas of warm SST anomalies are seen in the northern hemisphere Atlantic Ocean.

An advantage of using Joules as the climate metric of global heat changes is that an adequately sampled snapshot at any moment of time is all that is needed to monitor the heating within the climate system. Unlike surface air temperature by itself (that has been the main climate metric used to assess global warming), in which there is a lag between a radiative imbalance and an equilibrium temperature; e.g. see Equation 1-1 in NRC 2005: Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties ), there is no lag between a radiative imbalance and the amount of Joules in the climate system.

We can, therefore, apply an assessment of the current anomalies in ocean heat content to determine where the global warming signal is most pronounced. Data provided by the European Centre for Medium Range Forecasting of near-current ocean heat content anomalies (presented as ocean temperature anomalies) can be used to illustrate the current spatial complexity of the ocean heating. The ECMWF presents data for the following slices through the oceans:

1. Equatorial depth temperature anomaly

2. Latitude –depth temperature anomaly at 165E

3. Latitude –depth temperature anomaly at 140W

4. Latitude –depth temperature anomaly at 109.7W

5. Latitude –depth temperature anomaly at 30W

The near-surface temperature anomaly (5 m depth) is also available from the ECMWF.

There is an issue as to whether all of the important spatial scales of the heat anomalies are sampled in these analyses. Nonetheless, there are several important conclusions from even a cursory examination of these slices even if we still need improved spatial monitoring.

A significant portion of the warming is at depth. The portion of this heat that is a depth below the thermocline is not readily available to heat the atmosphere above or to contribute to enhanced evaporation of water vapor from the ocean surface. This heat is “sequestered” for an unknown period of time.

The anomalies have significant horizontal, as well as vertical variations. Such horizontal structure could be a result of the heterogeneous character of a number of the climate forcings, as we discussed in the weblog entry for July 28, 2005 (What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?) and/or related to the complexity of ocean dynamical and thermodynamic processes. A number of these anomalies are cooler than the long-term average.

Thus, the answer to the question posed in this weblog is that global warming has significant spatial variations. Global warming is not a more-or-less uniform warming spread across the oceans. Such a spatially complex warming pattern further supports the claim that a multiple set of climate forcings, in addition to the more homogeneous radiative forcing of the well-mixed greenhouse gases, is altering our climate. The reconstruction of the observed temporal evolution of the spatial pattern over the last several decades by the global climate models remains an unrealized goal.

Comment on Real Climate Response as to “What is a First-Order Climate Forcing?”

A very useful dialog on the issue as to what is a first order climate forcing was initiated by Real Climate. The comment can be read on their web site at the URL listed above. My response, which I posted as a comment on their website is,

“Gavin-thank you for commenting on my Climate Science weblog (http://www.climatesci.org//). With respect to what is a first order climate forcing, the perspective of the 2005 National Research Council report (http://www.nap.edu/openbook/0309095069/html/) includes the following

“Regional variations in radiative forcing may have important regional and global climatic implications that are not resolved by the concept of global mean radiative forcing. Tropospheric aerosols and landscape changes have particularly heterogeneous forcings. To date, there have been only limited studies of regional radiative forcing and response. Indeed, it is not clear how best to diagnose a regional forcing and response in the observational record; regional forcings can lead to global climate responses, while global forcings can be associated with regional climate responses. Regional diabatic heating can also cause atmospheric teleconnections that influence regional climate thousands of kilometers away from the point of forcing. Improving societally relevant projections of regional climate impacts will require a better understanding of the magnitudes of regional forcings and the associated climate responses.”
(http://www.nap.edu/books/0309095069/html/5.html)

and

“Several types of forcings-most notably aerosols, land-use and land-cover change, and modifications to biogeochemistry-impact the climate system in nonradiative ways, in particular by modifying the hydrological cycle and vegetation dynamics. Aerosols exert a forcing on the hydrological cycle by modifying cloud condensation nuclei, ice nuclei, precipitation efficiency, and the ratio between solar direct and diffuse radiation received. Other nonradiative forcings modify the biological components of the climate system by changing the fluxes of trace gases and heat between vegetation, soils, and the atmosphere and by modifying the amount and types of vegetation. No metrics for quantifying such nonradiative forcings have been accepted. Nonradiative forcings have eventual radiative impacts, so one option would be to quantify these radiative impacts. However, this approach may not convey appropriately the impacts of nonradiative forcings on societally relevant climate variables such as precipitation or ecosystem function. Any new metrics must also be able to characterize the regional structure in nonradiative forcing and climate response.” (http://www.nap.edu/books/0309095069/html/6.html)

Although these climate forcings may not alter the global mean surface temperature, they are first order climate forcings in terms of their substantial role in influencing the climate system including the planetary atmospheric circulation. We both agree that the radiative effect of carbon dioxide, methane and sulphates are first order climate forcings. What we need to do now is discuss what are the criteria that we use to apply the term “first order”. The title of the National Research Council report “Radiative Forcing of the Climate System: Expanding the Concept and Addressing Uncertainties” clearly indicates that we need to move beyond the current perspective of referring to global averaged temperature as the primary metric to assess human caused climate change, and of “CO2, CH4 and sulphates (the main non-soot aerosol)” as “the only ‘first order’ climate forcings”. I discuss this subject further on my weblog, and welcome further discussion of this issue.

On “the conclusion was made that the ‘balance of evidence’ supported the notion of ongoing human-caused climate change”, the evidence of a human fingerprint on the global and regional climate is incontrovertible as clearly illustrated in the National Research Council report and in our research papers (e.g., see http://blue.atmos.colostate.edu/publications/pdf/R-258.pdf).”

Are Today’s Droughts Exceptional?

We published a paper this year in Pure and Applied Geophysics entitled “Drought 2002 in Colorado – An unprecedented drought or a routine drought?“. We concluded that the recent drought was not exceptional and that the state is actually more vulnerable to drought than it was in the past as a result of population growth and increased competition for water among different sectors of society.

New research on droughts in other locations in the country from the National Climate Data Center (NCDC) provides additional evidence that the recent droughts over the last few years are not exceptional. One study in Montana based on proxy data from tree rings for the period 1540-2000 (see Climate of 2005 – August Montana Drought) has the following conclusions,

“Comparison of the MSD (mean summer deficit) reconstruction with the instrumental records of summer drought for northwest Montana leads to several observations. First, the most severe single-year summer droughts of the 20th century were probably matched or exceeded on only a few occasions in the prior four centuries. Second, the cumulative deficit during the extended period of dry summers in the 1920s and 1930s appears to be unprecedented in the 461-year reconstruction. This period was one of rapid retreat of glaciers in Glacier National Park (Pederson et al. in press). The current drought, though it includes two very dry summers, pales in comparison with that event and many other extended drought events seen in the tree ring record.”

Pederson, G. T., S. T. Gray, D. B. Fagre, and L. J. Graumlich, in press. “Long-Duration Drought Variability and Impacts on Ecosystem Services: A Case Study from Glacier National Park, Montana USA.” Ecological Applications. in press.

For Illinois, the following conclusions have been presented (see Climate of 2005 – August
Illinois Drought):

“The current drought was very unusual in the speed of its onset, with PDSI (Palmer Drought Severity Index) dropping from slightly wet to severe drought in just five months. In the Illinois Division 1 instrumental record, this has occurred only once before, in 1936. But the drought’s current severity has been matched or exceeded on numerous occasions in the past, according to both the instrumental and tree-ring records. ”

All existing evidence has shown that earlier historic droughts and paleo-droughts in the United States clearly were of greater severity than what has occurred recently. This should be sobering to any climate scientist and policymaker. Even without human climate change, we are vulnerable to severe social, economic, and environmental disruption. This is why we have proposed the vulnerability paradigm as a more effective framework to address our future risks as has been discussed in our July 19, August 16, August 31, and September 6 weblogs.

A New Integrated Climate/Environmental Activity: the Northern Eurasia Earth Science Partnership Initiative (NEESPI)

The recognition that environmental, including climate, issues require an integrated assessment is being implemented by NEESPI. This is an international effort including USA and Russian scientific involvement. The Science plan states,

“Climatic changes in Northern Eurasia (20% of the global land mass) interact and affect the rate of the Global Change through atmospheric circulation and through strong biogeophysical and biogeochemical feedbacks. These feedbacks arise from changes in surface energy, water, and carbon budgets of the continent. How this carbon-rich, cold region component of the Earth system functions as a regional entity and interacts with and feeds back to the greater global system is to a large extent unknown. Thus, the capability to predict future changes that may be expected to occur within this region and the consequences of those changes with any acceptable accuracy is currently uncertain and hampers projections of the Global Change rates.

One of the primary reasons for this lack of regional Earth system understanding is the relative paucity of well-coordinated, multidisciplinary and integrating studies of the critical physical and biological systems. Furthermore, the critical measurements needed to monitor changes in the area are not available. NEESPI strives to understand how the land ecosystems and continental water dynamics in northern Eurasia interact with and alter the climate system, the biosphere, the atmosphere, and the hydrosphere of the Earth. Its overarching Science Question is: How do we develop our predictive capability of terrestrial ecosystems dynamics over Northern Eurasia for the 21st century to support global projections as well as informed decision making and numerous practical applications in the region?

The foci of the NEESPI research strategy are the deliverables, which support both national (primarily the U.S. Climate Change Science Program, CCSP) and international science (e.g. International Geosphere Biosphere Program, IGBP) programs.”

The NEESPI mission statement reads,

“The Northern Eurasia Earth Science Partnership Initiative (NEESPI) will identify the critical science questions and establish an international program of coordinated research on the state and dynamics of terrestrial ecosystems in northern Eurasia and their interactions with the Earth’s Climate system to enhance scientific knowledge and develop predictive capabilities to support informed decision-making and practical applications. ”

This program builds on the recognition that climate is a system involving physical, biological and chemical processes, as we discussed in our July 11th blog entitled “What is climate? Why does it matter how we define climate?”, and as identified by the 2005 National Research Council report , and the International Geosphere-Biosphere Programme.

More details on NEESPI are available at http://neespi.org/.

Comment on Webster et al. September 16, 2005 Science Article “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment”

The September 16,2005 article by Webster et al. in Science concludes that there has been a large increase in the number and proportion of hurricanes reaching Saffir-Simpson category 4 and 5 hurricanes over the past decade. They report that these increases have taken place while the number of tropical cyclones and tropical cyclone days has decreased in all basins except the North Atlantic.

This is a clearly written article by very well-respected scientists. There are, however, several substantive issues with the study. First, an informative figure illustrating the maximum potential for hurricanes as a function of SST was described by a 1988 paper by Robert Merrill entitled “Environmental Influences on Hurricane Intensification” (see Figure 2 in that paper). This research was completed for the Atlantic hurricane region, but the SST thresholds should be the same for the other basins. As presented in that paper, Category 4 and 5 hurricanes require sea surface temperatures (SST) of over 27°Celsius. Thus the criteria that should be examined are anomalies in SST that result in increases of temperatures above the 27°C criteria. Category 5 hurricanes require temperatures 28°C. Has the area of SST above these thresholds increased, for example?

The Webster et al. Science article actually present a range of SST values during the respective hurricane seasons for the different hurricane basins in Figure 1 of their paper. These range from around 29.5°C for the north Indian Ocean to around 27.5°C for the north Atlantic and eastern Pacific Ocean basins. Such an analysis suggests that regardless of SST temperature trends, the north Indian Ocean should have a greater porportion of Category 4 and 5 hurricanes. Clearly, there are other factors besides SST that determine the ability of the tropical cyclones to attain Category 4 and 5 intensities as we discussed in Pielke, R.A., Jr. and R.A. Pielke, Sr., 1997: Hurricanes: Their nature and impacts on society. John Wiley and Sons, England, 279 pp, and Pielke, R.A., 1990: The hurricane. Routledge Press, London, England, 228 pp. Indeed, it is rare for the hurricane to attain its maximum intensity due to other limitiations. The Science article is silent on the relation between the different SSTs in the different hurricane regions with respect to the proportion that reach category 4 and 5 intensities.

The major limitations that prevent hurricanes from reaching their full potential includes vertical wind shear, dry air intrusion, and less than optimal outflow aloft in the upper portion of the hurricane circulation. In idealized hurricane modeling it is relatively easy to create hurricanes that attain their maximum intensity, since these limitations are not prescribed in the model initialization or boundary conditions. In the real world, however, one or more of these limitations almost always exists (fortunately!). Hurricane Katrina is an example where a particularly effective outflow aloft, moist tropical air, and a lack of vertical wind shear, along with the elevated SSTs, pemitted the cyclone to attain a category 5 intensity.

In Nicholls, M.E., and R.A. Pielke, 1995: A numerical investigation of the effect of vertical wind shear on tropical cyclone intensification. 21st Conference on Hurricanes and Tropical Meteorology, AMS, Boston, April 24-28, Miami, Florida, 339-341, we investigated the role of shear on hurricane intensification. In Eastman, J.L., M.E. Nicholls, and R.A. Pielke, 1996: A numerical simulation of Hurricane Andrew. Second International Symposium on Computational Wind Engineering, 4-8 August 1996, Fort Collins, CO , we investigated the skill in the simulation of a Category 4 and 5 hurricane. A clip of our model simulation of Hurricane Georges is available from Video Clip of Hurricane Georges (8 Megabytes). We suggest that the use of high resolution models of hurricane intensification as influenced by SST anomolies should be a high priority in addressing the issue of their role, relative to other influences, on hurricane intensification. It is only with fine-scale hurricane model simulations of real world systems, that are able to resolve the eyewall region of the hurricane, that we can adequately address the issue of the relative role of the spatial pattern and magnitude of SSTs on the intensity that they attain.

As another issue, why use 5-year running averages? Tropical cyclones respond to the SST that exists when they occur. The analysis should have correlated tropical cyclone intensity with the specific SST values for each event. The conclusions of the authors would be more robust if they evaluated the Category 4 and 5 hurricanes on a case by case basis with respect to the ocean SST temperatures and SST anomolies over which the hurricanes moved.

Finally, the same analysis, as shown by Pat Michaels (Global Warming and Hurricanes: Still No Connection), when applied to an earlier time period (starting in 1945) than in the Webster et al. Science study, indicates that a high proportion of Category 4 and 5 hurricanes also occurred then. Webster et al. is clear as to why they chose to use the more recent era with the better data coverage. However, coverage for the Atlantic basin, for instance, is quite good since 1945 and should have been assessed against the more recent time period. The Michaels communication ideally should have been submitted to Science as a comment, so that Webster et al. would need to respond. Nonetheless, it highlights an important issue that needs to be resolved as to whether Webster et al. are analyzing the upward portion of a cyclic behavior of hurricane intensities or a real much longer-term trend.

Webster et al. do appear to recognize this issue. The Science article concludes with the statement (referring to the trend towards more frequent and intense hurricanes),

“This trend is not inconsistent with recent climate model simulations that a doubling of CO₂ may increase the frequency of the most intense tropical cyclones, although the attribution of the 30-year trends to global warming would require a longer global data record and, especially, a deeper understanding of the role of hurricanes in the general circulation of the atmosphere and ocean, even in the present climate state”.

This qualification of their work was lost when the news media highlighted in their reports (e.g., see “Experts say global warming is causing stronger hurricanes“).

The National Oceanographic and Atmospheric Administration (NOAA) provides a very valuable current assessment of SST anomolies , which can be directly related to the SST temperature anomolies presented in the Webster et al. paper. For example, for the September 17th data, above average SST temperatures in the Atlantic Ocean hurricane region is evident, as is the cooling to below average where the recent hurricanes have traveled. The analysis also shows a complex spatial pattern of SSTs which further supports the need for the Webster et al conclusions to be assessed with respect to the actual SSTs traversed by the hurricanes. The NOAA data also show that the hurricane region exceeds the threshold for Category 4 and 5 hurricanes, even without additional warming.

Additional Evidence as to Why Land-Use/Land-Cover Change is a First-Order Climate Forcing

A September 13, 2005 NASA Press release entitled “Tropical Deforestation Affects Rainfall in the U.S. and Around the Globe” reports on recent research work led by Roni Avissar of Duke University that has provided further evidence of the importance of land-use/land-cover change as a first-order climate forcing. As the news release states

“Today, scientists estimate that between one-third and one-half of our planet’s land surfaces have been transformed by human development… Our study carried somewhat surprising results, showing that although the major impact of deforestation on precipitation is found in and near the deforested regions, it also has a strong influence on rainfall in the mid and even high latitudes,” said Roni Avissar, lead author of the study, published in the April 2005 issue of the Journal of Hydrometeorology… Deforestation does not appear to modify the global average of precipitation, but it changes precipitation patterns and distributions by affecting the amount of both sensible heat and that released into the atmosphere when water vapor condenses, called latent heat,” said Avissar. ‘Associated changes in air pressure distribution shift the typical global circulation patterns, sending storm systems off their typical paths.’ And, because of the Amazon’s location, any sort of weather hiccup from the area could signal serious changes for the rest of the world like droughts and severe storms.”

This work is discussed in detail in Avissar et al. 2005.

This new research supports the conclusions summarized in the NASA publication entitled Local or Global Problem? . As reported in that publication with respect to the global climate implications of land-use/land-cover change,

“Though their results drew national media attention from many sources, all the scientists involved in the research agree that the scientific arena is where the results should be evaluated. Pielke hopes these results will convince scientists to give the land cover-climate connection more attention. In the past, he has been frustrated by the lack of attention to the topic.

Gordon Bonan is a climate modeler for the National Center for Atmospheric Research in Boulder, Colorado. `It’s definitely true that historically, the emphasis in global climate change research has been on other climate forcings-greenhouses gases, solar variability, aerosols-and that the role of land cover has been neglected. Roger’s work, his persistence, has really played a large role in bringing people around to the importance of it.’ Bonan thinks people are finally beginning to listen.

So far, what research has been done on the global-scale influence of land cover change on climate seems to suggest it plays a minor role. That’s not surprising, says Bonan, considering how small the Earth’s land surface is compared to its oceans and that our most common metric for climate change is global mean temperature. Even significant changes in the temperature where we live can get ‘washed out’ (at least for a while) in the global average of a world mostly covered by oceans.

‘Nobody experiences the effect of a half a degree increase in global mean temperature,’ Bonan says. ‘What we experience are the changes in the climate in the place where we live, and those changes might be large. Land cover change is as big an influence on regional and local climate and weather as doubled atmospheric carbon dioxide-perhaps even bigger.’ That’s the idea Pielke says he has been trying to get across for years. ‘Climate change is about more than a change in global temperature,’ he says. ‘It’s about changes in weather patterns across the Earth.’ Even if it turns out that land cover change doesn’t significantly alter the globally-averaged surface temperature of the Earth, it’s still critically important. ‘The land is where we live. This research shows that the land itself exerts a first order [primary] influence on the climate we experience.’

The full text of this May 17 2005 NASA publication article is available at the link above.