On May 22 the National Oceanic and Atmospheric Administration announced its forecast that a very active hurricane season is looming in the North Atlantic region. For the six-month 2006 North Atlantic hurricane season beginning June 1, NOAA is predicting 13 to 16 named storms, with eight to 10 becoming hurricanes, of which four to six could become “major” hurricanes of Category 3 strength or higher. The hurricane forecast announcement does not mention research that has linked global warming to increased hurricane intensity.
Federal hurricane experts say the Atlantic Ocean is in a cycle of intense hurricane activity that started in 1995, producing 28 named storms last year, more than at any other time since the government began keeping records in the mid-1800’s.
Other experts, including some scientists working for the oceanic agency, say that a decades-long global warming trend, linked by most climate experts to the accumulation of carbon dioxide and other heat-trapping emissions, may be playing a role in increasing the power of hurricanes in recent decades.
The May 22 NOAA hurricane forecast announcement does not discuss the question of the relationship between hurricanes and global warming. A May 1 news release reported on recent research at the NOAA Geophysical Fluid Dynamics Lab suggesting that greenhouse gas emissions from human activity may contribute significantly to ocean warming in the region of the tropical Atlantic where many hurricanes originate. The news release reporting these findings does not mention research that has linked the sea surface warming to observed and projected increases in hurricane intensity. Text of the May 1 NOAA news release:
AREA WHERE HURRICANES DEVELOP IS WARMER, SAY NOAA SCIENTISTS
The region of the tropical Atlantic where many hurricanes originate has warmed by several tenths of a degree Celsius over the 20th century, and new climate model simulations suggest that human activity, such as increasing greenhouse gas emissions into the atmosphere, may contribute significantly to this warming. This new finding is one of several conclusions reported in a study by scientists at NOAAs Geophysical Fluid Dynamics Laboratory in Princeton, N.J., published today in the Journal of Climate.
“This very long-term increase in temperature may seem small, but is comparable in magnitude to shorter time-scale, multi-decadal changes that many scientists now believe contribute strongly to an increase in hurricane activity in the Atlantic,” said Thomas Knutson, lead author of the paper and a senior research meteorologist at GFDL. “The challenge is to understand the relative roles of anthropogenic and natural factors in producing these temperature changes—and this study is a step in that direction – and then to determine whether and how these long-term changes in temperature could be affecting Atlantic hurricane activity.”
The region, which extends from 10N to 20N in the area of the Cape Verde Islands, has been identified as the origin for a large portion of major hurricanes in the tropical North Atlantic, and is known as the “Main Development Region.” Ocean surface temperatures in this region warmed over the 20th century, roughly tracking the global mean, or average, but this region has greater multi-decadal variability than the global mean does when looking at long-term trends.
The climate model simulations are based on a new state-of-the-art coupled atmosphere-ocean model developed over several years at GFDL. The new simulations include improved representations of a number of environmental factors that can affect climate, such as greenhouse gases, volcanic eruptions, solar variability, land-use changes and atmospheric aerosols, very fine particulate matter in the air. More research is being conducted to improve the representation of these forcings, and of the aerosol effects in particular.
The new model simulations used current best estimates of a number of historical climate forcings to simulate climate variations over the 20th century. In the Main Development Region, the observed warming during the 20th century is simulated much more realistically in the models that include anthropogenic forcing than in models with only natural effects.
The results suggest that the century-scale warming tendency in the Main Development Region may have been caused largely by anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations.
Other sources of anthropogenic forcing include aerosols and land-use changes. Examples of natural effects are volcanic emissions, long-term variability of solar radiation, and internal variability, such as the internal processes within the climate system.
Atlantic hurricane variability and its causes is the subject of intensive scientific investigations by scientists. Earlier studies suggest that warmer tropical sea surface temperatures can lead to hurricanes of greater intensity. Other studies at NOAA have concluded that warmer sea surfaces is one of several important factors affecting Atlantic hurricane activity. Ongoing research continues to address uncertainties in the observed trend, past climate forcing estimates, internal variability and climate sensitivity.
Other significant conclusions of the new GFDL study include findings that extend beyond the North Atlantic. Simulations of global trends and trends in several other specific regions also produced more realistic results when anthropogenic forcing was included in addition to natural effects. An example is the Indian Ocean and western tropical Pacific, where a regional warming trend has emerged particularly clearly during the past half century. These conclusions support similar findings from earlier studies….