Is
the Climate Changing?
Indeed It Is
By Michael MacCracken Director of the Office of the
US Global Change Research Program, Washington DC and Tom Karl Senior Scientist
at the National Climatic Data Center of the National Oceanic and Atmospheric
Administration in Asheville NC 
As published in Bob Ryan's 1997 Almanac & Guide for the
Weatherwise published by WRC-TV, Washington DC
Weather
is what is happening in the atmosphere right now--the instantaneous
state of the atmosphere--and it is constantly changing. It is a thunderstorm
on Monday at 4:30 p.m., a hurricane on July 12, a snowstorm on January
20. Weather is what actually happens. Weather forecasters try to predict
the exact sequences of what will happen. Satellite and surface observations
and better computer models are helping make these predictions useful
out to almost a week in advance. Because the atmosphere is so turbulent
and chaotic, however, predicting the detailed sequence of weather events
out more than about two weeks will likely never be possible.
In contrast, climate is the frequency or probability of various types of weather occurring over an extended time period and geographical region. By changing the focus from predictions of the sequence and timing of weather events to statistical predictions about the state of the atmosphere, climate predictions are able to overcome many of the problems related to the chaotic nature of the weather (although other difficulties arise). Mathematicians typically require a few decades of data to get stable statistics (even longer periods are used for data on extreme events). This time is conveniently also about the length of a human generation -- and maybe of our weather memories. So, when parents tell their children winters were harsher or milder when they were growing up, they are saying that the weather they experienced then was different from the weather they expect today--and this change we call a change in climate.
Although we have mathematical ways to express the difference between weather and climate, the old saying is really true: "Climate is what you expect, and weather is what you get!" All the discussion about climate change concerns whether human activities or natural events (or both) have changed and are going to change the climate in the future. Thus, the central questions are: "Is the climate we are experiencing different from what our parents/grandparents experienced and different from the climate our children/grandchildren are likely to experience? If so, why is this happening and what will this mean?"
HAS THE CLIMATE BEEN CHANGING?
Analyzing the record for Washington D.C., the average annual temperature has increased about 0.5oF from the period 1956-75 to the period 1976-95, with winter temperatures increasing slightly more than summer temperatures. The amount of precipitation has also increased by about 4%, or nearly 2 inches per year. The number of very heavy precipitation events (greater than 2 inches in one day, which usually happens about once or twice per year) has increased by 8%, and the number of snowfalls of more than one inch per year has decreased from an average of just over 6 events per year down to about 4 or 5 per year.
The records for particular periods and locations are not always representative. The changes in Washington DC could be just a fluctuation or could be due to urbanization. So a better way to answer whether the climate is changing is to look at the trend over a larger region and over a longer period. For the US east of the Mississippi, there was little change in temperature from the period 1956-75 to 1976-95 because some regions warmed and some cooled. But precipitation, which may represent weather changes over a larger region, has increased by about 4%. Longer records indicate that the temperature has decreased slightly (by about 0.1oF from 1901-47 to 1948-95) while precipitation has increased by 4%. The proportion of precipitation derived from very heavy precipitation events has also increased by a few percent during this period.
To get a more general picture, scientists also look at as much of the globe as they can, mostly the land areas and the shipping lanes. These analyses provide important information about global climate.
1. Globally, the average temperature of the Earth has warmed about 1oF since the mid-19th century when measurements began. This may seem small, but it is about the difference, in the Northern Hemisphere, between the present and the colder period when Hans Brinker skated on his silver skates around the Dutch canals in winter--small changes in average temperatures have had large social consequences.
2. The global temperature is warmer now than at any time since at least 1400 AD, which is as far back as scientists have good estimates of temperatures from direct and indirect observations. Fragmentary records suggest the Earth is warmer than it has been in many thousands of years, maybe nearly 100,000 years.
3. The recent warming shows up both in records of air temperature and in indirect physical and biological evidence. Mountain glaciers are melting, springtime snow cover is reduced, the temperature of the ground has been increasing, and sea levels have risen by several inches this century.
WHY ARE ALL THESE CHANGES OCCURRING AND HOW DO WE KNOW THIS?
Figuring this out is a scientific detective story because there are several potential culprits. The world may be warming because: (1) the Sun is getting brighter; (2) a natural fluctuation is occurring; or (3) human activities such as emissions of carbon dioxide, methane and other greenhouse gases are enhancing the Earth's natural greenhouse effect that keeps the Earth relatively warm. The most likely factor can be determined by looking at the "climatic fingerprints" of each possibility: (1) More solar radiation would warm the surface and the lower and upper atmosphere--yet the upper atmosphere is cooling and the changes in the Sun's radiation are small; (2) Natural fluctuations would warm some regions and cool others and warm at some times and cool at others--yet the warming has been very prolonged and the warming has a distinctive pattern; and (3) The best estimates from climate models suggest that the climate changes from human-induced factors add up to just about the expected amount of change, and the distinctive geographical and vertical patterns of temperature change generally match those from the increased concentrations of greenhouse gases and sulfate particles (mainly from coal combustion) and the decreased stratospheric ozone from CFCs. This combination of factors led the UN-sponsored Intergovernmental Panel on Climate Change (IPCC), in their 1995 international scientific assessment, to conclude that "the balance of evidence suggests that there is a discernible human influence on global climate."
WHAT CLIMATE CHANGES CAN WE EXPECT IN THE FUTURE?
Over the past 200 years, emissions from cars, power plants, and other human activities have led to about a 30% increase in the natural concentration of carbon dioxide and more than a doubling of the atmospheric concentration of methane. As the world population grows and the world develops and uses more coal, oil, and natural gas, the concentrations of these gases are projected to increase even more sharply.
As this happens, the warming influence of greenhouse gases will become an even more dominant influence on climate. Scientific assessments suggest that the global average temperature a hundred years from now is likely to be about 4oF warmer than today (the predicted range is from about 1.5 to 6oF due to uncertainties in scientific understanding and in projections about future population and technologies.) Changes in precipitation and a sea level rise of about 20 inches are also predicted. Unless there are drastic cutbacks in emissions of greenhouse gases, significant further changes are likely in the 22nd century and beyond.
WHAT DO WE EXPECT THESE CLIMATE CHANGES WILL MEAN FOR US?
Such large climate changes would certainly have consequences. Warmer temperatures will increase heat stress and would allow tropical diseases to spread into middle latitudes. Rising temperatures will also reduce heating bills, but increase the need for air conditioning. Earlier springtime warming will help farming in some regions, but hotter and drier summers will likely reduce agricultural production elsewhere. Changes in storm systems and a rising snowline will affect water supply and storage systems. Rising sea levels will require building levees to protect cities, retreating from coastlines, and abandoning some islands. Changing temperatures and precipitation patterns will disrupt forest and grassland systems, and sea level rise will inundate some wetlands. The world will not end, but conditions will change significantly from those to which we and nature have become accustomed.
WHAT IS HAPPENING ABOUT THIS? WHAT CAN EACH OF US DO?
Coal, oil, and natural gas provide more than 80% of the world's energy and there is currently no completely cost-effective alternative--but much still can be done. The world's governments have signed a climate convention and are negotiating steps that can be taken internationally. Meanwhile, there are steps we can take in this country. Three important steps are to use energy more efficiently, to choose products that reduce energy use, and to participate in recycling. Additional steps are to encourage development and use of less energy intensive products, to support scientific research to find out more about what might happen, and to develop less costly alternative energy technologies (for example, wind, solar, and biomass).
The stakes are high. If we continue to increase atmospheric concentrations of greenhouse gases at projected rates and our present understanding of the climate system is about right, the changes over the next century could transform the Washington area's climate into that of Charleston (SC) and Maine's climate to that of this area. Summer vacations in the mountains of New England would be different, and summertime agriculture in the southern Great Plains might not be possible. While we have all adapted to almost unnoticed climate changes since we were young, and we can readily move around the country and into air-conditioned buildings, the prospective rate of climate change will be much faster than we have experienced in the past.
This is all a lot for us and for international negotiators to consider. But, human activities are starting to affect our "Spaceship Earth"--we are experiencing the consequences of our choices and our lifestyles.
In contrast, climate is the frequency or probability of various types of weather occurring over an extended time period and geographical region. By changing the focus from predictions of the sequence and timing of weather events to statistical predictions about the state of the atmosphere, climate predictions are able to overcome many of the problems related to the chaotic nature of the weather (although other difficulties arise). Mathematicians typically require a few decades of data to get stable statistics (even longer periods are used for data on extreme events). This time is conveniently also about the length of a human generation -- and maybe of our weather memories. So, when parents tell their children winters were harsher or milder when they were growing up, they are saying that the weather they experienced then was different from the weather they expect today--and this change we call a change in climate.
Although we have mathematical ways to express the difference between weather and climate, the old saying is really true: "Climate is what you expect, and weather is what you get!" All the discussion about climate change concerns whether human activities or natural events (or both) have changed and are going to change the climate in the future. Thus, the central questions are: "Is the climate we are experiencing different from what our parents/grandparents experienced and different from the climate our children/grandchildren are likely to experience? If so, why is this happening and what will this mean?"
HAS THE CLIMATE BEEN CHANGING?
Analyzing the record for Washington D.C., the average annual temperature has increased about 0.5oF from the period 1956-75 to the period 1976-95, with winter temperatures increasing slightly more than summer temperatures. The amount of precipitation has also increased by about 4%, or nearly 2 inches per year. The number of very heavy precipitation events (greater than 2 inches in one day, which usually happens about once or twice per year) has increased by 8%, and the number of snowfalls of more than one inch per year has decreased from an average of just over 6 events per year down to about 4 or 5 per year.
The records for particular periods and locations are not always representative. The changes in Washington DC could be just a fluctuation or could be due to urbanization. So a better way to answer whether the climate is changing is to look at the trend over a larger region and over a longer period. For the US east of the Mississippi, there was little change in temperature from the period 1956-75 to 1976-95 because some regions warmed and some cooled. But precipitation, which may represent weather changes over a larger region, has increased by about 4%. Longer records indicate that the temperature has decreased slightly (by about 0.1oF from 1901-47 to 1948-95) while precipitation has increased by 4%. The proportion of precipitation derived from very heavy precipitation events has also increased by a few percent during this period.
To get a more general picture, scientists also look at as much of the globe as they can, mostly the land areas and the shipping lanes. These analyses provide important information about global climate.
1. Globally, the average temperature of the Earth has warmed about 1oF since the mid-19th century when measurements began. This may seem small, but it is about the difference, in the Northern Hemisphere, between the present and the colder period when Hans Brinker skated on his silver skates around the Dutch canals in winter--small changes in average temperatures have had large social consequences.
2. The global temperature is warmer now than at any time since at least 1400 AD, which is as far back as scientists have good estimates of temperatures from direct and indirect observations. Fragmentary records suggest the Earth is warmer than it has been in many thousands of years, maybe nearly 100,000 years.
3. The recent warming shows up both in records of air temperature and in indirect physical and biological evidence. Mountain glaciers are melting, springtime snow cover is reduced, the temperature of the ground has been increasing, and sea levels have risen by several inches this century.
WHY ARE ALL THESE CHANGES OCCURRING AND HOW DO WE KNOW THIS?
Figuring this out is a scientific detective story because there are several potential culprits. The world may be warming because: (1) the Sun is getting brighter; (2) a natural fluctuation is occurring; or (3) human activities such as emissions of carbon dioxide, methane and other greenhouse gases are enhancing the Earth's natural greenhouse effect that keeps the Earth relatively warm. The most likely factor can be determined by looking at the "climatic fingerprints" of each possibility: (1) More solar radiation would warm the surface and the lower and upper atmosphere--yet the upper atmosphere is cooling and the changes in the Sun's radiation are small; (2) Natural fluctuations would warm some regions and cool others and warm at some times and cool at others--yet the warming has been very prolonged and the warming has a distinctive pattern; and (3) The best estimates from climate models suggest that the climate changes from human-induced factors add up to just about the expected amount of change, and the distinctive geographical and vertical patterns of temperature change generally match those from the increased concentrations of greenhouse gases and sulfate particles (mainly from coal combustion) and the decreased stratospheric ozone from CFCs. This combination of factors led the UN-sponsored Intergovernmental Panel on Climate Change (IPCC), in their 1995 international scientific assessment, to conclude that "the balance of evidence suggests that there is a discernible human influence on global climate."
WHAT CLIMATE CHANGES CAN WE EXPECT IN THE FUTURE?
Over the past 200 years, emissions from cars, power plants, and other human activities have led to about a 30% increase in the natural concentration of carbon dioxide and more than a doubling of the atmospheric concentration of methane. As the world population grows and the world develops and uses more coal, oil, and natural gas, the concentrations of these gases are projected to increase even more sharply.
As this happens, the warming influence of greenhouse gases will become an even more dominant influence on climate. Scientific assessments suggest that the global average temperature a hundred years from now is likely to be about 4oF warmer than today (the predicted range is from about 1.5 to 6oF due to uncertainties in scientific understanding and in projections about future population and technologies.) Changes in precipitation and a sea level rise of about 20 inches are also predicted. Unless there are drastic cutbacks in emissions of greenhouse gases, significant further changes are likely in the 22nd century and beyond.
WHAT DO WE EXPECT THESE CLIMATE CHANGES WILL MEAN FOR US?
Such large climate changes would certainly have consequences. Warmer temperatures will increase heat stress and would allow tropical diseases to spread into middle latitudes. Rising temperatures will also reduce heating bills, but increase the need for air conditioning. Earlier springtime warming will help farming in some regions, but hotter and drier summers will likely reduce agricultural production elsewhere. Changes in storm systems and a rising snowline will affect water supply and storage systems. Rising sea levels will require building levees to protect cities, retreating from coastlines, and abandoning some islands. Changing temperatures and precipitation patterns will disrupt forest and grassland systems, and sea level rise will inundate some wetlands. The world will not end, but conditions will change significantly from those to which we and nature have become accustomed.
WHAT IS HAPPENING ABOUT THIS? WHAT CAN EACH OF US DO?
Coal, oil, and natural gas provide more than 80% of the world's energy and there is currently no completely cost-effective alternative--but much still can be done. The world's governments have signed a climate convention and are negotiating steps that can be taken internationally. Meanwhile, there are steps we can take in this country. Three important steps are to use energy more efficiently, to choose products that reduce energy use, and to participate in recycling. Additional steps are to encourage development and use of less energy intensive products, to support scientific research to find out more about what might happen, and to develop less costly alternative energy technologies (for example, wind, solar, and biomass).
The stakes are high. If we continue to increase atmospheric concentrations of greenhouse gases at projected rates and our present understanding of the climate system is about right, the changes over the next century could transform the Washington area's climate into that of Charleston (SC) and Maine's climate to that of this area. Summer vacations in the mountains of New England would be different, and summertime agriculture in the southern Great Plains might not be possible. While we have all adapted to almost unnoticed climate changes since we were young, and we can readily move around the country and into air-conditioned buildings, the prospective rate of climate change will be much faster than we have experienced in the past.
This is all a lot for us and for international negotiators to consider. But, human activities are starting to affect our "Spaceship Earth"--we are experiencing the consequences of our choices and our lifestyles.
