Day 2 Question 2: Hurricane intensity debate

Question 2:

The very active 2004 and 2005 Atlantic hurricane seasons have helped fuel the debate about global warming and hurricanes. Summarize the debate. Point to problems with each side's opinion. Pick a side and defend it. Explain how we might address the question with the available data. Be creative.

Introduction

Presently a debate rages on as to whether the recent upswing in hurricane intensity is within the natural variability of the climate system or being driven by the secular warming of the Earth’s atmosphere and oceans. One group of scientists feel that the observed increase in tropical cyclone intensity around the globe is being driven by warmer sea-surface temperatures (SSTs) which are a direct result of the atmospheric loading of carbon dioxide gas by humans. Another group of scientists are arguing that natural cycles of ocean circulation affect the amount and intensity of tropical cyclones in the Atlantic basin (hurricanes). A third possibility is that both of these explanations are valid, and that the recent strengthening of storms is part of a natural cycle that is being enhanced by global warming. I will present all three of these possibilities, strengths and weaknesses in their arguments, and then present my own opinion in the concluding remarks.

Background

Tropical cyclones, better known in the United States as hurricanes, which is the name given to tropical cyclones that form in the North Atlantic, Carribean Sea, Gulf of Mexico, or Eastern Pacific, may be thought of as convective engines of latent heat, or Carnot cycles, as a first approximation (Emanuel, 1987). In this simplified view, global warming can theoretically alter the maximum potential intensity (MPI) of tropical cyclones through escalating the surface energy balance and the upper-level cold air return, both of which intensify the cyclone (Henderson-Sellers et al., 1998).

Over the past 30 years, tropical SSTs have been steadily rising. This is generally attributed to anthropogenic global warming (Emanuel, 2005). In this same period, the number and percent of tropical cyclones intensifying to categories 4 and 5 increased (Webster et al., 2005). The relative increases in the number of category 4 and 5 storms for the 15-year periods 1975-1989 and 1990-2004 are shown in Figure 1 for different ocean basins, while the percent of cyclones reaching these intensities are shown in Figure 2 for the same basins and periods. Interestingly, while the number of strong tropical cyclones has increased in all basins, the absolute number of storms (not shown) has decreased in all basins except the North Atlantic since 1995 (Webster et al., 2005). In the last 10 years, there has been an increase in the frequency and intensity of hurricanes in the North Atlantic, while over the last 30 years, there has been a 50% increase in the intensity and duration of hurricanes with no increase in the frequency (Emanuel, 2005).

Figure 1: Changes in the numbers of tropical cyclones reaching category 4 or 5 for the 15-year periods of 1975-1989 and 1990-2004 for different ocean basins (adapted from Webster et al., 2005).

Figure 2: Changes in the percents of tropical cyclones reaching category 4 or 5 for the 15-year periods of 1975-1989 and 1990-2004 for different ocean basins (adapted from Webster et al., 2005).

Potential causes

1. Global Warming

Global climate models (GCMs) reveal a significant increase in the potential intensity of hurricanes with anthropogenic warming (Emanuel, 1987; Henderson-Sellers et al., 1998). This leads to the natural prediction that hurricane intensities should increase in the future. Given the current estimated warming in the tropics of 0.5°C, the resulting changes in hurricane intensities should not yet be detectable (Emanuel, 2005). There is no consistent link between hurricane frequencies and global warming, nor has any long-term trend in hurricane frequency been established (Emanuel, 2005).

Still, an obvious and strong relationship (r2 = 0.69) exists between SSTs and the total power dissipated (power dissipation index, PDI) by hurricanes each year (Emanuel, 2005), as shown in Figure 3. The strong rise in tropical SSTs over the last 30 years, which has been attributed to anthropogenic warming, is well-correlated with the near doubling of hurricane PDI in the same period. That evidence, coupled with the well-known relationships between SSTs and hurricane intensities (e.g. Emanuel, 1987), suggests that anthropogenic warming is at least partially responsible for this observed increase in the destructiveness of hurricanes.

Figure 3: Annual PDI for the North Pacific and North Atlantic basins compared to annually averaged tropical SSTs (source: Emanual, 2005).

Further evidence for the relationship between hurricane intensity and global warming comes from modeling studies. Knutson and Tuleya (2004) demonstrate that in a CO2-enriched atmosphere, the number of intense tropical cyclones predicted by circulation models increases, while the number of storms peaking out at weaker intensities decreases. They get similar results irrespective of the model and the sub-grid-scale convective parameterization scheme they use. Their results are summarized in Figure 4, which shows histograms of hurricane intensity results from all 1296 experiments. The open circles represent the control runs, while the filled circles represent the high CO2 cases. Note that there is no increase in hurricane frequency with high CO2, but the frequency of more intense storms does increase. This matches what is presently observed (Emanuel, 2005).

Figure 4: Hurricane intensity (mb) frequencies from 1296 modeling experiments. Open circles represent the control run, while filled circles represent the high CO₂ experiments (source Knutson and Tuleya, 2004).

Knutson and Tuleya attribute the consistent increase in hurricane intensity to a correlated increase in convective available potential energy (CAPE), despite the expected and observed cooling of the upper troposphere. These results coincide with observations made by Gettelman et al. (2002) which show increased CAPE measured from radiosonde data in the tropics at most of the 15 stations monitored, especially since 1975. Similarly, their modeled increase in precipitation intensity from the storms coincides with GCM simulations (e.g. IPCC, 2001) and the understanding that warmer air has a higher saturation vapor pressure. The relationship between temperature and saturation vapor pressure is dictated by the Clausius-Clapeyron equation (Oke, 1987; Stull, 1988), but this relationship is non-linear and may not include positive feedbacks which occur with the release of latent heat in convective storms (Trenberth et al., 2003).

The robustness of Knutson and Tuleya’s results lends great support for the notion that global warming can cause tropical cyclones to intensify, but it cannot show a unequivocal link to the current climb in hurricane strength. Moreover, Henderson-Sellers et al. (1998) indicate that modeling results which simulate tropical cyclones must be viewed carefully, as GCMs have too coarse a resolution to realistically capture the factors which dictate cyclone intensity. While making the causal jump between warmer SSTs and more intense hurricanes seems logical, and the balance of observations support this relationship, Trenberth (2005) concedes that a definitive linkage is not presently possible. Likewise, Peilke et al. (2005) argue that due to the complexity of the climate system, drawing a certain relationship between global warming and tropical cyclone intensity is a bit precocious before a clear relationship is demonstrated and widely accepted.

2. Natural Cycles

While the connections between global warming and increased hurricane intensity are clear to some, others argue that this upswing in storminess is likely part of a 60-70 year cycle which alters the strength with which ocean currents cycle warm water and heat around the globe. These researchers believe that we are now in the fast-flowing, or warm mode of this cycle, which has been referred to by some as the Atlantic Multidecadal Oscillation (AMO; Kerr, 2000). In this fast phase, warm Atlantic SSTs and low vertical wind-shear favor tropical cyclogenesis.

The AMO was first recognized by Delworth and Mann (2000), who showed that multidecadal oscillations in temperature proxy data extend back through the entire 330 year record. Using two independent coupled ocean-atmosphere models, they were able to produce comparable fluctuations which involved the oscillation of the speed of the thermohaline circulation (THC) in the North Atlantic. The THC is the ocean conveyor belt in which a continuous flow of warm surface waters is drawn from the tropical waters northward into the North Atlantic. As it cools near Greenland, and becomes saltier due to salt loading from sea ice, the water becomes more dense and sinks. The cool waters then flow deep in the ocean back to the original sources and rise. It is the sinking in the North Atlantic, we believe, that controls the speed of the conveyor.

We now understand that the AMO is a natural cycle in the speed of the THC (Knight et al., 2005) driven by years of cool winds off Canada cooling the waters in the North Atlantic, causing them to sink faster, driving the ocean conveyor faster. This faster conveyor then drives warmer waters into the North Atlantic, leaving cooler waters in the South Atlantic. These conditions drive easterly waves off of Africa, provide warmer SSTs in the North Atlantic, and lessen vertical wind-shear between the upper- and lower-troposphere, all of which are conditions favorable for tropical cyclone formation (Goldberg et al., 2001).

Goldberg et al. (2001) argue that the observed multidecadal variability in hurricane activity is greater than what is predicted from global warming. They further indicate that while tropical SSTs have increased gradually over the last 100 years, Atlantic hurricane activity has exhibited multidecadal cycles. Henderson-Sellers et al. (1998) point out that while there have been several studies of the potential effects of global warming on the frequency of Atlantic hurricanes, the results do not agree. Furthermore, it can be argued that the present active hurricane epoch (1995-present) only appears more active than the previous active epoch (1926-1970) because of more advanced monitoring systems including satellite technology.

3. Could it be both?

One final explanation for the upswing in tropical cyclones is a combined effect of the natural AMO cycle and global warming. This point is illustrated in Figure 5, a simple cartoon-like graphic depicting the smoothed global temperature record of the 20th century (green), which is known to correlate with the AMO, a natural 70-year cycle to represent the AMO (purple), and a 0.6°C/century warming trend (orange; IPCC, 2001). This graphic is strictly for illustrative purposes, but demonstrates that natural cycles and global warming may work in concert to produce the recent climatic fluctuations we have observed. In fact, if the purple and orange lines are added, they faithfully create the green line.

Figure 5: Cartoon graph of 20^th century temperatures (heavily smoothed; green), and the break-down of that curve into a cycle with an approximate period of 70 years (representing the AMO; purple) and a long-term trend (representing global warming; orange). This illustrates the possibility that both the AMO and global warming contributed to the observed temperatures of the 20^th century, and presumably to hurricane intensity cycles.

While Figure 5 is strictly meant to illustrate a point, it implores an additional question that has been posed recently by several researchers. The climate system exhibits a number of oscillations on a variety of time scales, ranging from glacial-interglacial cycles of 100ka to the 2 year QBO, and all points in between. As we shift between warm and cold phases of the PDO, for example, some anticipate that the next warm phase will be warmer than the previous one, as each is now riding on the back of a significant global warming trend. If this is true of all cycles, then will the next warm phase of the AMO potentially be even more devastating than any of the previous ones?

What’s really going on?

While there is strong evidence for both global warming and the AMO contributing to hurricane intensities, an additional piece of the puzzle is currently coming into the spotlight. A rapid freshening of the North Atlantic waters over the past 40 years was recently observed (e.g. Dickson et al., 2002). This freshening, predicted by models of global warming (IPCC, 2001), is potentially caused by enhanced precipitation, river runoff, and increased melt of glaciers and sea ice, especially the Greenland ice sheet. A fear of this freshening is that it may cause a slowing of the THC, and therefore of meridional heat transport, especially to Europe (Quadfasel, 2005; Sutton and Hodson, 2005). This freshening may be responsible for a recent slow down of the Atlantic conveyor as observed at 25°N, a common proxy for THC circulation (Bryden et al., 2005).

This latest finding adds support to the anthropogenic forcing of global warming, as climate models have predicted human-induced warming to cause a slow-down of the THC. Additionally, those who argue that natural cycles are responsible for the upswing in tropical cyclone intensity are left to explain how the THC is slowing down when they claim that we are in the fast mode of the AMO. One final note is that the AMO does not explain, through adequate physical principles, how tropical cyclone intensities are increasing around the globe (see Figures 1 and 2, for example).

With all of this evidence, I would have to stand firmly on the side of those who blame global warming for the increase in storminess. As for the notions that natural cycles have persistently forced hurricane intensity and that global warming is not predicted to have such a strong influence on tropical storm intensity with such a modest warming, my response is simply to restate a famous quote:

Due to the complexity of the climate system, and the abruptness of climate changes that punctuate paleoclimate records, we cannot expect climate to behave exactly as models tell us. I believe that we have reached a tipping point in climate and the plethora of changes we are currently witnessing is merely the beginning.

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