Our Warming Planet
Not all of that energy would be absorbed by the Earth. Roughly 30 percent of the total solar energy that strikes the Earth is reflected back into space by clouds, atmospheric aerosols, reflective ground surfaces, and even ocean surf. The remaining 70 percent is absorbed by the land, air, and the oceans. The absorbed light is mostly in the form of ultraviolet, visible, and near-infrared solar radiation.
Absorption of solar energy heats up our planet’s surface and atmosphere and makes life on Earth possible. The energy does not stay bound up in the Earth’s environment forever. If it did, then the Earth would grow hotter and hotter until its temperature exceeded that of the sun. Instead, as the rocks, the air, and the sea heat, they emit thermal radiation. Much of this thermal radiation, which is largely in the form of longwave infrared energy, travels directly out into space, leaving the Earth and allowing it to cool. Such radiation is invisible to our eyes, but our hands can feel it radiating from a fire or a car engine.
Some of this outgoing longwave infrared radiation, however, is re-absorbed by water vapor, carbon dioxide, and other greenhouse gases in the atmosphere and is then re-radiated back toward the Earth’s surface. On the whole this re-absorption process is good. If there were no greenhouse gases or clouds in the atmosphere, the Earth’s average surface temperature would be a very chilly -18°C (-0.4°F) instead of the comfortable 15°C (59°F) that it is today.
What has many people worried now is that over the past 250 years humans have been artificially raising the concentration of greenhouse gases in the atmosphere. Our factories, power plants, and cars burn coal and gasoline and spit out a seemingly endless stream of carbon dioxide. We produce millions of pounds of methane by allowing our trash to decompose in landfills and by breeding large herds of methane-belching cattle. Nitrogen-based fertilizers, which we use on nearly all our crops, release unnatural amounts of nitrogen oxide into the atmosphere.
Once these carbon-based greenhouse gases get into the atmosphere, they stay there for decades or longer. According to the Intergovernmental Panel on Climate Change (IPCC), since the industrial revolution, carbon dioxide levels have increased 31 percent and methane levels have increased 151 percent. Paleoclimate readings taken from ice cores and fossil records show that these gases, two of the most abundant greenhouse gases, are at their highest levels in the past 420,000 years. Many scientists fear that the increased concentrations of greenhouse gases have prevented additional thermal radiation from leaving the Earth. In essence, these gases are trapping excess heat in the Earth’s atmosphere in much the same way that a windshield traps solar energy that enters a car.
Much of the available climate data appear to back these fears. Temperature data gathered from many different sources all across the globe show that the surface temperature of the Earth, which includes the lower atmosphere and the surface of the ocean, has risen dramatically over the past century. The IPCC estimates the increase has been between 0.4°C and 0.8°C. Worldwide measurements of sea level show a rise of 0.1 to 0.2 meters over the last century. Readings gathered from glaciers reveal a steady recession of the world’s continental glaciers. Taken together, all of these data suggest that over the last century the planet has experienced the largest increase in surface temperature in 1,000 years.
As of now, greenhouse gases afford a plausible explanation for such changes. In the Earth’s distant past, drastic increases in carbon dioxide nearly always coincide with large increases in Earth surface temperatures. Conversely, ice ages are almost always accompanied by a decrease in carbon dioxide.
Logic dictates that, as third world nations develop their economies and first world nations consume more energy, greenhouse gas concentrations will continue to rise. Though scientists have not reached a consensus, most leading researchers and organizations purport that the average surface temperature of the Earth will increase along with increasing emissions. According to the IPCC, the surface temperature could rise by between 1.4°C and 5.8°C by the end of the century. Scientists at the Goddard Institute for Space Studies, NASA’s division spearheading climate modeling efforts, report that we should expect between 0.5°C and 1°C over the next 50 years.
At first glance, these numbers probably do not seem threatening. After all, temperatures typically change a few degrees whenever a storm front moves through. Such temperature changes, however, represent day-to-day regional fluctuations. When surface temperatures are averaged over the entire globe for extended periods of time, it turns out that the average is remarkably stable. Rarely in the Earth’s history has the average surface temperature changed as dramatically as the changes that scientists are predicting for the next century. During the last ice age 20,000 years ago, for instance, the Earth was roughly 5°C cooler than it is today. Since then it has warmed up, although not steadily, to present levels. That’s an increase of roughly 1°C every 4,000 years. Current global warming scenarios predict, at the bare minimum, a 1°C increase over the next century.
Global Warming and Hurricanes
Fig. 1. Comparison of simulated hurricane intensities. (more)
The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth(i)s climate is warmed by increasing levels of greenhouse gases in the atmosphere. Although we cannot say at present whether more or fewer hurricanes will occur in the future with global warming, the hurricanes that do occur near the end of the 21st century are expected to be stronger and have significantly more intense rainfall than under present day climate conditions. This expectation (Figure 1) is based on an anticipated enhancement of energy available to the storms due to higher tropical sea surface temperatures.
The results shown in Figure 1 are based on a simulation study carried out by Thomas R. Knutson and Robert E. Tuleya at NOAA(i)s Geophysical Fluid Dynamics Laboratory (GFDL). In this study hurricanes were simulated for a climate warming as projected to occur with a substantial build-up of atmospheric CO2. An increase of intensity of about one-half category on the Saffir-Simpson scale was simulated for an 80 year build-up of atmospheric CO2 at 1%/yr (compounded).
New Observational Studies
Recently, two studies have been published which argue that hurricane intensities may have already increased markedly in recent decades as the tropical oceans have warmed. Emanuel (Nature, Aug. 4, 2005) reports that a measure of the power dissipated by tropical cyclones (proportional to the cube of wind speeds accumulated over the North Atlantic and western North Pacific basins) has approximately doubled since about 1950, with most of the increase occurring over the past 30 years. According to Emanuel, increases in both intensity and duration of tropical cyclones have contributed to this apparent increase. Emanuel(i)s power dissipation index (PDI) is strongly correlated with sea surface temperatures in these basins, which have increased markedly over the same period. For an update, see this discussion. A critique of Emanuel(i)s Atlantic results by Chris Landsea appears in the 22 December 2005 issue of Nature. Landsea(i)s reanalysis of the Atlantic PDI record indicates that there is no evidence for a trend in the Atlantic basin-wide PDI (1949-2004), in contrast to Emanuel(i)s original Figure 1. In his reply in the same issue, Emanuel accepts Landsea(i)s revisions to his Atlantic analysis but maintains that there is still a global warming signal emerging in the multi-basin indices.
In a second study, Webster et al. (Science, Sept. 16, 2005) report that the number of category 4 and 5 hurricanes has almost doubled globally over the past three decades. Although their analysis spans a shorter time period than Emanuel(i)s, their results indicate that a substantial increase has occurred in all six tropical storm basins. A key question for these studies is the quality and degree of homogeneity in the hurricane intensity data, a subject of debate in the hurricane research community.
It should be noted that the rate of increase of hurricane intensities implied in Emanuel(i)s results (per degree of SST warming) is much greater than that simulated in our future projections. We are not yet able to reconcile these large differences in apparent sensitivity of the tropical cyclone intensities. We speculate that these discrepancies could arise from three sources: i) possible overestimation of the observed intensity trends; ii) possible underestimation by our model of the sensitivity of tropical cyclone intensities to SST changes; or iii) possible influence of related environmental variables such as trends in atmospheric temperatures (lapse rates) and moisture. Further investigation is ongoing.
Fig. 2. Tropical storm simulations. (more)
Background and Previous Studies
An increase in the upper-limit intensity of hurricanes with global warming was suggested on theoretical grounds by M.I.T. Professor Kerry Emanuel in 1987. In the late 1990s, Knutson, Tuleya, and Kurihara at GFDL/NOAA began simulating samples of hurricanes from both the present-day climate and from a greenhouse-gas warmed climate. This was done by "telescoping-in" on coarsely resolved tropical storms in GFDL(i)s global climate model using the high-resolution GFDL hurricane prediction model (Figure 2). A research report describing this work was published in the Feb 13, 1998 issue of Science, with a more detailed paper in Climate Dynamics (1999, vol. 15). All of these studies, as well as our more recent ones, include the moderating effect of atmospheric stabilization aloft under high CO2 conditions, rather than simply increasing the sea surface temperature alone.
In a follow-up study, which appeared in the Journal of Climate (June 2001), NOAA scientists Knutson and Tuleya teamed up with Isaac Ginis and Weixing Shen of the University of Rhode Island to explore the climate warming/ hurricane intensity issue using hurricane model coupled to a full ocean model. The coupled model was used to simulate the "cool SST wake" generated by the hurricanes as they moved over the simulated ocean (Figure 3). The model simulations including this additional feedback still showed a similar percentage increase of hurricane intensity under warm climate conditions as the original model without ocean coupling.
Fig. 3. Sea surface temperatures and sea level pressure. (more)
The most recent and comprehensive study by Knutson and Tuleya, published in Journal of Climate in September 2004 (download paper), confirms the general conclusions of previous studies but makes them more robust by using future climate projections from nine different global climate models and four different versions of the GFDL hurricane model. The GFDL hurricane model used for the study is an enhanced resolution version of the model used to predict hurricanes operationally at NOAA(i)s National Centers for Environmental Prediction. According to this latest study, an 80 year build-up of atmospheric CO2 at 1%/yr (compounded) leads to roughly a one-half category increase in potential hurricane intensity on the Saffir-Simpson scale and an 18% increase in precipitation near the hurricane core. A 1%/yr CO2 increase is an idealized scenario of future climate forcing. As noted by the Intergovernmental Panel on Climate Change (IPCC), there is considerable uncertainty in projections of future radiative forcing of earth(i)s climate. A criticism of our paper by Michaels et al. was recently published in the Journal of Climate. Our response is available here.
An implication of these studies is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse-gas induced warming may lead to an increasing risk in the occurrence of highly destructive category-5 storms."