Hurricanes and Climate Change
Definition of a Hurricane
A hurricane is a type of tropical cyclone, which is a general term for a low-pressure system with activity like thunder and lightning that develops in the tropics or subtropics, between about 30 degrees north and 30 degrees south latitude. In the Northern Hemisphere, these storms rotate counter-clockwise. In the Southern Hemisphere, they rotate clockwise. Stronger systems are called “hurricanes” or “typhoons,” depending on where they form. Weaker tropical cyclones might be called “tropical depressions” or “tropical storms.”
It’s unclear whether climate change will increase or decrease the number of hurricanes, but warmer ocean surface temperatures and higher sea levels are expected to intensify their impacts.
Hurricanes are subject to various climate change-related influences. Warmer sea surface temperatures could intensify tropical storms wind speeds, potentially delivering more damage if they make landfall. Based on sophisticated computer modeling, scientists expect a 2-11 percent increase in average maximum wind speed, with more occurrences of the most intense storms. Rainfall rates during these storms are also projected to increase by about 20 percent.
In addition, sea level rise is likely to make future coastal storms, including hurricanes, more damaging. Globally averaged, sea level is expected to rise by 1-4 feet during the next century, which will amplify coastal storm surge. For example, sea level rise intensified the impact of Hurricane Sandy, which caused an estimated $65 billion in damages in New York, New Jersey, and Connecticut in 2012, and much of this damage was related to coastal flooding.
The connection between climate change and hurricane frequency is less straightforward.
Globally, the number of tropical storms that form each year ranges between 70 and 110, with about 40 to 60 of these storms reaching hurricane strength. But records show large year-to-year changes in the number and intensity of these storms. Some models project no change or a small reduction in the frequency of hurricanes by 2100 while others show an increase in frequency.
It’s important to note that changes in frequency and intensity vary from basin to basin. In the North Atlantic Basin, the long-term (1966-2009) average number of tropical storms is about 11 annually, with about six becoming hurricanes. More recently (2000-2013), the average is about 16 tropical storms per year, including about eight hurricanes. This increase in frequency is correlated with the rise in North Atlantic sea surface temperatures, which could be partially related to global warming.
One trend analysis published in the journal Nature shows the strongest hurricanes have also increased in intensity over the past two or three decades in the North Atlantic and Indian Oceans. Other areas in the Pacific and Indian Oceans show virtually no significant trends. Other trend analyses that include all hurricanes globally are similarly inconclusive, with upward trends in the North Atlantic and Indian Oceans and no apparent increase in frequency or intensity in other basins.
For the continental United States in the Atlantic Basin, models project a 75 percent increase in the frequency of category 4 and 5 hurricanes despite a possible decrease in the total frequency of all storms.
The tracks of tropical storms near the United States. Tropical storms are shown with red lines and major hurricanes (Category 3 and higher) are yellow. Source: National Hurricane Center. To see the legend, click here
Threats Posed by Hurricanes
The National Hurricane Center categorizes Atlantic hurricanes based on wind speed. A storm with winds exceeding 74 mph is a Category 1 hurricane. Storms with winds stronger than 111 mph are considered “major hurricanes” (Category 3 or higher). Many factors contribute to a hurricane’s impact, including its track, size, storm structure, rainfall amount, duration, and the vulnerability of the area it affects.
Eight of the 10 costliest hurricanes on record in the United States have occurred since 2004. Hurricanes Katrina (2005) and Sandy (2012) were by far the most damaging, costing $125 billion and $65 billion respectively. Hurricanes Andrew (1992) and Ike (2008) cost $27 billion each. Sandy ranks as the second most damaging storm or weather disaster since 1980, even though the storm was no longer a hurricane at landfall.
An important driver of the increased cost of hurricanes is increasing development in coastal areas. U.S. coastal populations grew by nearly 35 million people between 1970 and 2010. Coastal counties account for nearly 40 percent of the total U.S. population. As more development occurs in harm’s way — regardless of climate change — the more likely the damage will grow.
How to Build Resilience
Ways communities can bolster their resilience and reduce the impacts of hurricanes include:
- Preserving coastal wetlands and dunes to absorb storm surges.
- Replenishing beaches and improve infrastructure that affords coastal protection, such as seawalls.
- Elevating vulnerable buildings to reduce flood damage.
- Designing structures to be resilient to high winds and flying debris.
- Enacting policies that discourage development in vulnerable areas.
- Preparing prior to a storm’s arrival by boarding windows, clearing property of potential flying debris, and having an evacuation plan.
To Learn More
Extreme Heat and Climate Change
Across the globe, warming temperatures are increasing the risk of hot weather and decreasing the risk of colder weather. Hot summer days are getting hotter, as well as more frequent, sometimes stretching into multiday heat waves.
During the past decade, daily record high temperatures have occurred twice as often as record lows across the continental United States, up from a near 1:1 ratio in the 1950s. By midcentury, if greenhouse gas emissions are not significantly curtailed, scientists expect 20 record highs for every record low. The ratio could be 50:1 by the end of the century. By the 2050s, many of the Mid-Atlantic States including urban parts of Maryland and Delaware could see a doubling of days per year above 95 degrees F. In parts of the South, the frequency of days above 95 degrees F could triple, to over 75 days per year. While climate change will not mean the end of cold weather — scientists still expect record lows to occur in 2100 — the odds have clearly shifted (see map).
Extreme heat can also increase the risk of other types of disasters. When heat occurs in conjunction with a lack of rain, drought can occur. This, in turn, can encourage more extreme heat, as the sun’s energy acts to heat the air and land surface, rather than to evaporate water. Hot, dry conditions also increase the risk of wildfires, like the ones in 2013 in Colorado that were fueled by record heat and an ongoing drought.
More extreme heat in the 21st century. The map shows the projected increase in the number of days with maximum temperatures above 95°F for the later part of the 21st century. The color indicates the increase in the number of days (i.e., orange areas in Oklahoma are projected to experience 20 or more additional days of temperatures above 95°F). The hatching across most of the country indicates that the change passes a significance test in that location. Source: NOAA Technical Report NESDIS 142-9
Threats Posed by Extreme Heat
Extreme heat is the most deadly natural disaster in the U.S., killing on average more people (about 600 per year) than hurricanes, lightning, tornadoes, earthquakes, and floods combined. The Billion Dollar Weather Disasters database compiled by the National Oceanic and Atmospheric Administration lists heat waves as four of the top 10 deadliest U.S. disasters since 1980. Two heat waves in 1980 and 1988 were particularly deadly, contributing to 10,000 and 7,500 deaths respectively. These two events account for the vast majority of extreme weather-related deaths in the database, with Hurricane Katrina (1,833 deaths) as the only non-heat wave event that caused more than 500 fatalities.
High humidity and elevated nighttime temperatures (i.e., when nighttime low temperatures remain relatively warm) appear to be key ingredients in causing heat-related illness and mortality. Heat stress occurs in humans when the body is unable to cool itself effectively. Normally, the body can cool itself through sweating, but when humidity is high, sweat will not evaporate as quickly, potentially leading to heat stroke. When there’s no break from the heat at night, it can cause discomfort and lead to health problems, especially for the poor and elderly.
High temperatures at night can be particularly damaging to agriculture. Some crops require cool night temperatures, and heat stress for livestock rises when animals are unable to cool off at night. Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rates.
Higher summer temperatures will increase electricity use, especially during heat waves. An increase in cooling demand is already apparent over the past 20 years. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, make it difficult for power plants to comply with environmental regulations regarding their cooling water, and can make it harder to get permits for new facilities.
How to Build Resilience
Communities can bolster their resilience and reduce the impacts of extreme heat by:
- Creating heat wave preparedness plans, identify vulnerable populations, and open cooling centers during extreme heat.
- Using green roofs, improved building materials, and shaded building construction to reduce the urban heat island effect.
- Pursuing energy efficiency to reduce demand on the electricity grid, especially during heat waves.
- Shading and cooling livestock, breeding livestock selectively for heat tolerance, and switching to growing more heat-resistant crops.
To Learn More
A recent op-ed in the Wall Street Journal dredges up debunked conclusions drawn from a cherry-picked set of temperature measurements to try to call into question the reality and potential severity of climate change.
In a nutshell, authors Richard McNider and John Christy argue that warming in the upper atmosphere since 1979 is less than models had predicted and, therefore, models can’t be trusted and climate change shouldn’t be a concern.
In fact, virtually all climate data and research show that the Earth is warming. And it will continue to do so if we keep pumping greenhouse gases into the atmosphere. And this warming will bring an increased risk of more frequent and intense heat waves, higher sea levels, and more severe droughts, wildfires, and downpours.
To get at the facts, we can draw on recent climate assessments, including the State of the Climate report compiled by National Oceanic and Atmospheric Administration (NOAA), the Intergovernmental Panel on Climate Change (IPCC) Working Group I report, and the National Research Council’s America’s Climate Choices, plus other recent research (Thorne et al., 2011, Santer et al., 2013).
Based on this research, here are three things to keep in mind:
A lot of folks in the eastern half of the United States are breathing a sigh of relief that spring is just around the corner. Average temperatures this winter were among the Top 10 coldest in some parts of the Upper Midwest and South. More than 90 percent of the surface of the Great Lakes is frozen, the highest in 35 years.
But while East Coast and Midwest kids have been sledding and their parents have been shoveling, it has not been cold everywhere. In fact, many areas are unusually warm.
In Alaska, January temperatures were as high as they have been in 30 years. The Iditarod dogsled race was especially treacherous this month because of a lack of snow. Crews had to stockpile and dump snow on the ground at the finish line in Nome, where temperatures earlier this winter broke a record.
Globally, January was the fourth warmest on record – really – despite pockets of well-below-normal temperatures in parts of the United States. According to the National Oceanic and Atmospheric Administration (NOAA), most areas of the world experienced warmer-than-average monthly temperatures. For example:
- China experienced its second warmest January on record.
- France tied its warmest January.
- Parts of Brazil and Australia saw record heat.
January temperatures were above normal for much of the globe.
Most people at some point develop a “Plan B” – in case their first choice of college doesn’t accept them, or it rains on the day of their planned outdoor party, or the deal for the house they wanted falls apart. The same principle applies for more dire situations, such as a city having plans in hand for an orderly evacuation in case of a large-scale disaster. We hope such an event will never happen, but the mayor had better be prepared in case it does.
In a commentary today in the scientific journal Nature Climate Change, three colleagues and I discuss the need for a “Plan B” for climate change: How will we cope with increasingly severe climate impacts if we are unsuccessful in limiting global warming to a chosen target?
In the 2009 Copenhagen climate accord, countries set a goal of limiting global warming to below 2 °C (3.6 °F) above the average global temperature of pre-industrial times. However, given that the planet has already warmed by 0.8 °C, additional warming is already locked into the system, and global greenhouse gas emissions continue to rise, this “Plan A” has become increasingly difficult and may become impossible to achieve if widespread emissions reductions do not begin within this decade. A maximum warming target is a necessary goal of climate policy, but what if our efforts fall short?
Some voices in the environmental community will feel that asking this question is ceding failure, but I disagree. Instead, it means admitting that we can’t perfectly foresee the future and that we need to be prepared for surprises. This is called risk management and everyone from parents, to mayors, to companies, to the U.S. military uses risk management every day to cope with uncertainty.
As President Barack Obama prepares to deliver his State of the Union address, we believe it’s a good time to take a look at the state of our climate: the growing impacts of climate change, recent progress in reducing U.S. emissions, and further steps we can take to protect the climate and ourselves.
The consequences of rising emissions are serious. The U.S. average temperature has increased by about 1.5°F since 1895 with 80 percent of this increase occurring since 1980, according to the draft National Climate Assessment. Greenhouse gases could raise temperatures 2° to 4°F in most areas of the United States over the next few decades, bringing significant changes to local climates and ecosystems.
This week’s brief but bitter cold snap over more than half the country prompted intense discussion about the polar vortex ranging from educational to bombastic.
|Figure 1: A depiction of the “average” polar vortex on Jan. 6. The winds of the vortex correspond to the narrow “rainbow” areas. The map is an average of the upper atmosphere’s “topography” (specifically, the 500 millibar height) from all the January 6ths between 1980 and 2010.|
|Figure 2: The polar vortex on Jan. 6, 2014. The ridge (“R”) and trough (“T”) responsible for relatively warm weather in much of the West and bitterly cold weather in the Midwest and East have been labeled.|
So let’s be clear: The cold snap this week was unusual but not entirely unprecedented. A few super-cold days don’t disprove global warming, just like a day of rain doesn’t end a drought. At the same time, we don’t yet know whether climate change will change the odds of future outbreaks of bitter cold. Research is still underway, and as of now, we shouldn’t necessarily expect these events to be more or less frequent in future winters.
Here’s a Q&A to cut through the hype:
- What is the polar vortex? The polar vortex describes the air circulating aloft (thousands of feet above the ground) about the North Pole, and its extent is marked by a ribbon of strong winds that is often called the “jet stream.” (We most commonly focus on the North Pole, but a similar circulation is present around the South Pole, too).
In the map (Figure 1), which is from the point of view of the North Pole, the vortex corresponds to purple and blue colored areas. The band where the colors change from blue/purple to red/yellow indicates the location of the jet stream, or the outer edge of the vortex. Winds are strongest where this color gradient is tightly packed (e.g., over the Pacific Ocean and North Atlantic Ocean). It tends to be quite cold at the surface below the purple areas, and warmer under the red/yellow areas.
It’s important to note that this figure is an average of many winter days. On any given day, we would see a number of deviations from this average pattern.
- What happened this week? Comparing this week (Figure 2) to the average picture (Figure 1), we can see that the purple area of the vortex has contorted and moved farther south. Along with this pattern, there are substantial “wiggles” in the jet stream. These deviations in the circulation helped bring cold air into the continental United States that normally stays in northern Canada and the Arctic. Meteorologists look for these wiggles, called “ridges” and “troughs” (“R” and “T” on the map) when putting together a forecast. While the trough brought notable cold to the Midwest and the East, the ridge has kept parts of the West warmer than average and relatively dry (much to the dismay of skiers).
Joe Casola, staff scientist and program director of Science and Impacts, and Dan Huber, science and policy fellow, co-authored this article.
The terms “climate change” and “global warming” might conjure up images of balmy beaches and scalding deserts – a world without winter. But it’s more complicated than that.
As we prepare for the official arrival of the season on Dec. 21, let’s look at a few ways winters in the United States are changing because of global warming, and what we can do to adapt.
A year after Hurricane Sandy, more work remains to be done to help families and communities fully recover. But another pressing need, not only for those who were in Sandy’s wake but for all of us, is to learn from the storm’s devastating impacts and reduce the risk of future damage and loss of life.
Hurricane Sandy's estimated $65 billion in damages make it the second costliest hurricane in U.S. history, surpassed only by Hurricane Katrina.
Building resilience to the impacts of major coastal storms like Sandy—and to other types of extreme weather that are becoming more intense and frequent as a result of climate change—will require a commitment to better protect infrastructure and implement policies to help get people out of harm’s way. Both efforts should take into account how future sea level rise can amplify storm surges, potentially making future impacts greater than what we’ve experienced in the past.