GLOBAL WARMING: CAUSES CONSEQUENCES AND EFFECTS
CHAPTER ONE
INTRODUCTION
Weather is what's happening in the atmosphere on any given day, in a specific place. Local or regional weather forecasts include temperature, humidity, winds, cloudiness, and prospects for storms or other changes over the next few days. (Learn more about how NCAR studies weather.)
Climate is the average of these weather ingredients over many years. Some meteorologists like the saying that "climate is what you expect; weather is what you get," memorable words variously attributed to Mark Twain, Robert Heinlein, and others.
In practical terms, the climate for a particular city, state, or region tells you whether to pack short-sleeved shirts and shorts or parkas and mittens before you visit, while the local weather forecast tells you if you'll want to wear the parka by itself or with an extra sweater today.
Climate varies across space and time, so climate is studied on a variety of spatial and time scales.
To interpret today's atmospheric conditions, we need a reference period of average, or "normal," climate to compare it against. How long is long enough to define the average climate for a city, state, or region? The National Oceanic and Atmospheric Administration's National Weather Service calculates a 30-year average once a decade. The current "normals" (issued July 1, 2011) are based on data from 1981 to 2010. NOAA's FAQ helps put this metric in context. For example, it notes that "Normals were not designed to be metrics of climate change."
When it comes to climate on a global scale, the "normal" reference period depends on which climate components scientists want to study. For example, many scientists compare average global temperatures, precipitation, and other variables for the 20th and 21st centuries with the 30-year averages for 1870 to 1899, before major industrialization produced large quantities of greenhouse gas.
You can see how recent observations and future projections of warming and cooling compare to conditions at the end of the 19th century by watching a visualization of data from the NCAR-based Community Climate System Model in our Climate Change Multimedia Gallery.
Before thermometers
To understand how climate varies across time, scientists examine three kinds of climate data: observations, historical accounts, and environmental evidence locked up in fossils, ice cores, and other "proxy climate records."
Observations of temperature at Earth's surface date back as far as 350 years for some locations in England, but only about 100 to 150 years in most of the developed world. But even before the thermometer was invented, ancient civilizations kept records of droughts, floods, unusual hot or cold weather, and other climate indicators, including planting and harvest times.
While human accounts can take us back hundreds or thousands of years, we need other tools to understand how Earth's climate has varied during its much longer lifetime of about 4.5 billion years.
Paleoclimatology delves into the deep history of past climate variation through what are called "proxy records." Air bubbles trapped in ice cores, the composition of lake sediments, changes in tree rings, pollen fossils, and other parts of Earth's ancient environment have given scientists many clues to past temperature, precipitation, wind patterns, and the chemical composition of the atmosphere through time.
Observations, historic accounts, and paleoclimate data are used to test the reliability of computer models that simulate Earth's climate on time scales from decades, to centuries, to millennia. Studying prehistoric variations can also provide important clues about what to expect in a warmer world.
The Greenhouse Effect
The greenhouse effect is unquestionably real and helps to regulate the temperature of our planet. It is essential for life on Earth and is one of Earth's natural processes. It is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they effectively 'trap' heat in the lower atmosphere) and re-radiation downward of some of that heat. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C). So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect by the emission of greenhouse gases through fossil fuel combustion and deforestation. Without the so-called greenhouse gases, including carbon dioxide, methane, nitrous oxide, and water vapor, Earth would be too cold to inhabit. These gases in Earth's atmosphere absorb and emit heat energy, creating the greenhouse effect that keeps our planet's temperature livable.
Water vapor is the most plentiful greenhouse gas on the planet, accounting for about 60% of the current greenhouse effect. Even ozone helps trap some of the heat that makes life on Earth possible, but the "ozone hole" is a separate issue not directly related to global warming.
Too much of a good thing
Since the industrial revolution, people have burned vast amounts of coal, petroleum, and other fossil fuels to create heat and power. This releases carbon dioxide, the most plentiful human-produced greenhouse gas, into the atmosphere. The result: more heat is trapped in Earth's atmosphere instead of radiating out into space.
On average, carbon dioxide lasts more than a century in the atmosphere. As a result, CO2 is well mixed around the globe. Measurements collected atop Hawaii’s Mauna Loa and other locations show a steady rise in global carbon dioxide concentrations since 1958. These concentrations have increased by more than 35% since preindustrial times, according to the World Meteorological Organization. Other, less prevalent greenhouse gases have increased at different rates. Methane, for example, virtually leveled off after 1999 at 155% above its preindustrial level, but began climbing again in 2007.
The relationship between Earth's water cycle and global warming creates a well-known feedback loop. Warmer temperatures cause more water to evaporate from land and oceans into the atmosphere. The added water vapor then contributes to warmer temperatures, completing the feedback loop. This is just one of many feedbacks in the Earth system that climate scientists are studying to improve projections of future climate change.
Two scientists are credited with the discovery more than 100 years ago that increasing carbon dioxide in the atmosphere warms the entire planet: French researcher Jean Baptiste Fourier and Swedish scientist Svante Arrhenius. Their identification of what came to be called the greenhouse effect applies to both natural and human-produced additions of CO2.
As measurements of atmospheric CO2 levels showed steady increases after World War II (see What is the average global temperature now?), Earth system scientists looked for a corresponding rise in global average temperatures, basing their studies on the physical laws governing the greenhouse effect. By the early 1980s, climate scientists were calling this atmospheric response global warming. Not every place on Earth was expected to warm at the same rate, and rising temperatures were not the only impacts anticipated.
Boosting Earth's temperature and adding more acidity to the oceans creates wide-ranging effects that are changing all the "normal" weather and climate conditions on which we've based our agriculture, industry, and social systems (see What's the difference between climate and weather?). So some researchers talk about global climate change to convey that the situation is far more complex than temperature alone.
To some ears, "climate change" sounds less ominous than "global warming." However, the phrase was introduced by researchers not to minimize the situation but to convey the full scope of disturbances that can occur in association with changes in global temperature, such as changes in patterns of flood and drought.
Global Temperatures
Global surface temperatures have increased about 0.74°C (plus or minus 0.18°C) since the late–19th century, and the linear trend for the past 50 years of 0.13°C (plus or minus 0.03°C) per decade is nearly twice that for the past 100 years. The warming has not been globally uniform. Some areas (including parts of the southeastern U.S. and parts of the North Atlantic) have, in fact, cooled slightly over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Lastly, seven of the eight warmest years on record have occurred since 2001 and the 10 warmest years have all occurred since 1995.
Recent analyses of temperature trends in the lower and mid- troposphere (between about 2,500 and 26,000 ft.) using both satellite and radiosonde (weather balloon) data show warming rates that are similar to those observed for surface air temperatures. These warming rates are consistent with their uncertainties and these analyses reconcile a discrepancy between warming rates noted on the IPCC Third Assessment Report .
There are a few connections between the two, but they are largely separate issues.
First, it's important to know that ozone plays two different roles in the atmosphere. At ground level, "bad ozone" is a pollutant caused by human activities; it's a major component of health-damaging smog. The same chemical occurs naturally in the stratosphere, and this "good ozone" acts as a shield, filtering out most of the ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants. The Ennvironmental Protection Agency has a resource exploring these two roles: Ozone: Good Up High, Bad Nearby.
The ozone hole refers to the seasonal depletion of the ozone shield in the lower stratosphere above Antarctica. It occurs as sunlight returns each spring, triggering reactions that involve chlorofluorocarbons (CFCs) and related molecules produced by industrial processes. These reactions consume huge amounts of ozone over a few weeks' time. Later in the season, the ozone-depleted air mixes with surrounding air and the ozone layer over Antarctica recovers until the next spring. Other parts of the globe have experienced much smaller losses in stratospheric ozone.
Because of international agreements to li mit CFCs and related emissions instituted with the Montreal Protocol, it's expected that the ozone hole will be slowly healing over the next few decades.
The ozone hole does not directly affect air temperatures in the troposphere, the layer of the atmosphere closest to the surface, although changes in circulation over Antarctica related to the ozone hole appear to be changing surface temperature patterns over that continent. Ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect. The main greenhouse gas responsible for present-day and anticipated global warming, however, is carbon dioxide produced by burning of fossil fuels for electricity, heating, and transportation.
Higher up, the loss of stratospheric ozone has led to some cooling in that layer of the atmosphere. An even larger effect comes from carbon dioxide, which acts as a cooling agent in the stratosphere even though it warms the atmosphere closer to ground level. This paradox occurs because the atmosphere thins with height, changing the way carbon dioxide molecules absorb and release heat. Together, the increase in carbon dioxide and the loss of ozone have led to record-low temperatures recently in the stratosphere and still higher up in the thermosphere. Far from being a good thing, this cooling is another sign that increasing levels of carbon dioxide are changing our planet's climate.
Current state of our atmosphere and our planet
Climatologists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages. Between 1961 and 1990, the annual average temperature for the globe was around 57.2°F (14.0°C), according to the World Meteorological Organization.
In addition, we can evaluate climate over longer periods of observation. For example, in 2014, the global temperature was 1.24°F (0.69°C) above the long-term average for the 20th century, according to NOAA's National Climatic Data Center. That number made 2014 the warmest year on record in the NOAA database, which goes back to 1880.
Renewable sources of energy
Fossil fuels pour carbon dioxide and other greenhouse gases into the atmosphere whenever we burn these fuels to power our transportation, heat and light our homes, and keep our industries and other businesses running. Alternatives to coal, gasoline, heating oil, and other fossil fuels are being explored, improved, and put to use by the private sector and governments around the world, across a wide range of options.
In the United States, the National Renewable Energy Laboratory serves as a hub for research and development across the renewable energy spectrum, including
fuel from
- biomass
- photovoltaic’s
- wind
Energy savings from
- building construction and retrofitting
- electric infrastructure systems
- vehicle fuel-use technology
A recent search on a popular online search engine produced 23,900,000 results for "renewable energy"—just one indication that the global hunt for solutions to the fossil fuel problem is in full swing.
Numerous colleges and universities are also focusing research and course offerings on renewable energy questions, including many of UCAR's member, affiliate, and international affiliate universities. Since 2009, NCAR researchers have been developing methods to produce highly detailed, localized weather forecasts that are helping electric utilities pinpoint when wind power will be available.
Evidence of changes to the Earth's physical, chemical and biological processes is now evident on every continent.
To fully appreciate the urgency of climate change, it's important to understand the ways it affects society and the natural environment. Sea levels are rising and glaciers are shrinking; record high temperatures and severe rainstorms and droughts are becoming increasingly common. Changes in temperatures and rainfall patterns alter plant and animal behavior and have significant implications for humans. In this section, explore the connections between the climate data and the changes happening around you—and those you can expect to see in the future—in all parts of the globe, including your own backyard.
Not only are global warming-induced changes currently underway, but scientists also expect additional effects on human society and natural environments around the world. Some further warming is already unavoidable due to past heat-trapping emissions; unless we aggressively reduce today's emissions, scientists project extra warming and thus additional impacts.
The Climate Hot Map arranges current and future climate impacts into five main groupings:
- People
- Freshwater
- Oceans
- Ecosystems
- Temperature
Each of these major groupings, in turn, is divided into specific categories that describe more fully some of the consequences we may face. Click on any of the categories listed on the left for more information.
CHAPTER TWO
TEMPERATURE AND GLOBAL WARMING IN HUMAN COEXISTENCE TO INIMICAL DIMENSION
As our climate changes, the risk of injury, illness, and death from the resulting heat waves, wildfires, intense storms, and floods rise.
- Extreme heat. If high temperatures, especially when combined with high relative humidity, persist for several days (heat waves), and if nighttime temperatures do not drop, extreme heat can be a killer. Of all climate-related projections by scientists, rising temperatures are the most robust. Higher temperatures are also the most influenced by human behavior: the fewer heat-trapping emissions we release into the atmosphere, the cooler we can keep our planet. Because winter temperatures are rising faster than summer ones, cold-related deaths are likely to decline.
- "Natural" disasters. Projected changes in temperature and precipitation under global warming are likely to lead to other effects that threaten human health and safety. For example, changing precipitation patterns and prolonged heat can create drought, which can cause forest and peat fires, putting residents and firefighters in danger. However, a warming atmosphere also holds more moisture, so the chance of extreme rainfall and flooding continues to rise in some regions with rain or snow. In many heavily populated areas, sea-level rise is more likely to put people in the path of storm surges and coastal flooding. Warmer ocean waters may spawn more intense tropical hurricanes and typhoons while ocean cycles continue to be a factor in the frequency of tropical cyclones.
- Poor air quality. Three key ingredients—sunlight, warm air, and pollution from power plants and cars burning coal and gasoline—combine to produce ground-level ozone (smog), which humans experience as poor air quality. Higher air temperatures increase smog, if sunlight, fossil fuel pollution, and air currents remain the same.
- Allergens and other nuisances. Warmer temperatures and higher concentrations of carbon dioxide in the atmosphere stimulate some plants to grow faster, mature earlier, or produce more potent allergens. Common allergens such as ragweed seem to respond particularly well to higher concentrations of CO2, as do pesky plants such as poison ivy. Allergy-related diseases rank among the most common and chronic illnesses that can lead to lower productivity.
- Spreading diseases. Scientists expect a warmer world to bring changes in "disease vectors"—the mechanisms that spread some diseases. Insects previously stopped by cold winters are already moving to higher latitudes (toward the poles). Warmer oceans and other surface waters may also mean severe cholera outbreaks and harmful bacteria in certain types of seafood. Still, changes in land use and the ability of public health systems to respond make projecting the risk of vector-borne disease particularly difficult.
People do not bear the health risks of climate change equally because:
- Climate trends differ by region. People who live in floodplains, for example, are more likely to see river or coastal flooding. Similarly, people who live in regions with poor air quality today are at greater risk from poor air quality days in the future.
- Some people are more vulnerable to illness or death. Young children, the elderly, and those who are already ill are less able to withstand high temperatures and poor air quality, for example. Temperature extremes and smog hit people with heart and respiratory diseases, including asthma, particularly hard.
- Wealthy nations are more likely to adapt to projected climate change and recover from climate-related disasters than poor countries . Even within nations, less economically fortunate individuals are more vulnerable because they are less likely to have air conditioning and well-insulated homes, and because they have fewer resources to escape danger.
Better planning—through investments in infrastructure and public health strategies—can help communities become more resilient in a warming world. However, the costs of coping with health risks linked to severe climate change are often higher than the costs of curbing heat-trapping emissions in the first place. Food
Climate-related threats to global food production include risks to grain, vegetable, and fruit crops, livestock, and fisheries.
- Reduced yields. The productivity of crops and livestock, including milk yields, may decline because of high temperatures and drought-related stress.
- Increased irrigation. Regions of the world that now depend on rain-fed agriculture may require irrigation, bringing higher costs and conflict over access to water.
- Planting and harvesting changes. Shifting seasonal rainfall patterns and more severe precipitation events—and related flooding—may delay planting and harvesting.
- Decreased arability. Prime growing temperatures may shift to higher latitudes, where soil and nutrients may not be as suitable for producing crops, leaving lower-latitude areas less productive.
- More pests. Insect and plant pests may survive or even reproduce more often each year if cold winters no longer keep them in check. New pests may also invade each region as temperature and humidity conditions change. Lower-latitude pests may move to higher latitudes, for example.
- Risks to fisheries. Shifts in the abundance and types of fish and other seafood may hurt commercial fisheries, while warmer waters may pose threats to human consumption, such as increasing the risk of infectious diseases. Extreme ocean temperatures and ocean acidification place coral reefs-—the foundations of many of the world's fisheries-—at risk.
As with health risks, nations and individuals do not bear threats to the global food supply equally. Nations that lose arable land and critical fisheries may not have the resources or climate to pursue reasonable-cost options for maintaining food security. Some nations are also more vulnerable to unfavorable international trade agreements and regional strife that may interrupt food distribution
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