Earth is Changing

How is the global earth system changing?
Earth is currently in a period of warming. Over the last century, Earth's average temperature rose about 1.1°F (0.6°C). In the last two decades, the rate of our world's warming accelerated and scientists predict that the globe will continue to warm over the course of the 21st century. Is this warming trend a reason for concern? After all, our world has witnessed extreme warm periods before, such as during the time of the dinosaurs. Earth has also seen numerous ice ages on roughly 11,000-year cycles for at least the last million years. So, change is perhaps the only constant in Earth's 4.5-billion-year history.

Scientists note that there are two new and different twists to today's changing climate: (1) The globe is warming at a faster rate than it ever has before; and (2) Humans are the main reason Earth is warming. Since the industrial revolution, which began in the mid-1800s, humans have attained the magnitude of a geological force in terms of our ability change Earth's environment and impact its climate system.

Since 1900, human population doubled and then double again. Today more than 6.5 billion people inhabit our world. By burning increasing amounts of coal and oil, we drove up carbon dioxide levels in the atmosphere by 30 percent. Carbon dioxide is a "greenhouse gas" that traps warmth near the surface.

Humans are also affecting Earth's climate system in other ways. For example, we transformed roughly 40 percent of Earth's habitable land surface to make way for our crop fields, cities, roads, livestock pastures, etc. We also released particulate pollution (called "aerosols") into the atmosphere. Changing the surface and introducing aerosols into the atmosphere can both increase and reduce cloud cover. Thus, in addition to driving up average global temperature, humans are also influencing rainfall and drought patterns around the world. While scientists have solid evidence of such human influence, more data and research are needed to better understand and quantify our impact on our world's climate system.

What are the primary forcings of the Earth system?
The Sun is the primary forcing of Earth's climate system. Sunlight warms our world. Sunlight drives atmospheric and oceanic circulation patterns. Sunlight powers the process of photosynthesis that plants need to grow. Sunlight causes convection which carries warmth and water vapor up into the sky where clouds form and bring rain. In short, the Sun drives almost every aspect of our world's climate system and makes possible life as we know it.

Earth's orbit around and orientation toward the Sun change over spans of many thousands of years. In turn, these changing "orbital mechanics" force climate to change because they change where and how much sunlight reaches Earth. (Please see for more details.) Thus, changing Earth's exposure to sunlight forces climate to change. According to scientists' models of Earth's orbit and orientation toward the Sun indicate that our world should be just beginning to enter a new period of cooling -- perhaps the next ice age.

However, a new force for change has arisen: humans. After the industrial revolution, humans introduced increasing amounts of greenhouse gases into the atmosphere, and changed the surface of the landscape to an extent great enough to influence climate on local and global scales. By driving up carbon dioxide levels in the atmosphere (by about 30 percent), humans have increased its capacity to trap warmth near the surface.

Other important forcings of Earth's climate system include such "variables" as clouds, airborne particulate matter, and surface brightness. Each of these varying features of Earth's environment has the capacity to exceed the warming influence of greenhouse gases and cause our world to cool. For example, increased cloudiness would give more shade to the surface while reflecting more sunlight back to space. Increased airborne particles (or "aerosols") would scatter and reflect more sunlight back to space, thereby cooling the surface. Major volcanic eruptions (such as that of Mt. Pinatubo in 1992) can inject so much aerosol into the atmosphere that, as it spreads around the globe, it reduces sunlight and cause Earth to cool. Likewise, increasing the surface area of highly reflective surface types, such as ice sheets, reflects greater amounts of sunlight back to space and causes Earth to cool.

Scientists are using NASA satellites to monitor all of the aforementioned forcings of Earth's climate system to better understand how they are changing over time, and how any changes in them affect climate.

How does the Earth system respond to natural and human-induced changes?
Climate scientists have been monitoring Earth's energy budget since the 1978 launch of NASA's Nimbus-7 satellite. That mission carried a new instrument into space called the Earth Radiation Budget Experiment (or ERBE), designed to measure all of the energy leaving through the top of Earth's atmosphere. All of the incoming sunlight minus all of the reflected sunlight and emitted heat is our world's energy budget. The second law of thermodynamics compels Earth's climate system to seek equilibrium so that, over the course of a year the amount of energy received equals the amount of energy lost to space. So typically the global energy budget is in balance.
But the energy budget can tip out of balance in any of three ways:
a change in the amount of incoming solar radiation. Such a change can be either or both due to a change in the Sun's energy output, or the ongoing changes in Earth's orbital mechanics.
a change in the abundance of greenhouse gases in Earth's atmosphere. Increasing the concentration of gases like carbon dioxide and methane slows the rate at which Earth emits warmth back to space relative to the rate at which sunlight warms the surface.
a change in Earth's reflective features. Bright white objects reflect more sunlight than they absorb, whereas dark brown, dark green, and dark blue objects absorb more sunlight than they reflect. Thus, increasing the extent of reflective objects -- like clouds, aerosols, and ice sheets -- cools the Earth. Conversely, reducing the extent
While each of those types of changes can occur naturally, humans can influence only the latter two.
Earth's climate system is "sensitive" to any of the aforementioned changes at a scale of 1 watt per square meter, or greater. That amount of energy equals one Christmas tree light bulb for every 3-foot by 3-foot square of our world's surface. Climate scientists found that adding to or reducing Earth's energy budget by 1 watt per square meter is enough to cause the globe to warm or cool, depending upon the direction and magnitude of the change.

What are the consequences of change in the earth system for human civilization?
Earth's climate system has been remarkably stable over the last 20,000 years or so. Human civilization developed in that time span, and our world's average temperature warmed by about 5°C to the temperature it is today. This fact points to one of climate scientists' main concerns about global warming: the temperature is rising faster than at any other time in the history of human civilization and such rapid climate change is likely to seriously stress some populations who cannot adapt quickly enough to the changes.

There will be consequences as the globe warms, and these consequences depend upon the magnitude of the change. There will be both good and bad consequences as some populations may benefit from a warming globe even as others suffer from it. For example, farmers in some latitudes will experience a longer growing season, which may improve crop yields. However, scientists also observe for intense downpours of rain, causing floods, interspersed with longer periods of drought. So such erratic swings in the weather caused by global warming could negatively impact farmers' crop yields.

The warming will cause significant erosion of coastlines as sea level rises due to the heat-driven expansion of the ocean and the influx of freshwater runoff due to melting ice sheets and glaciers. This problem is serious because about 10 percent of the world's population lives in coastal areas level than 10 meters (30 feet) above sea level.
Arctic sea ice will continue to decline until we see a completely ice-free Arctic Ocean; some scientists say this could happen as soon as the year 2040. While this loss of Arctic ice may benefit commercial shipping due to the opening of the northwest passage, other species such as polar bears could face extinction.

A warming climate is likely to shift the ranges of certain types of invasive species (such as undesired weeds, Africanized bees, and fire ants) and infectious diseases (such as West Nile Virus, hantavirus, malaria, etc.). Access to clean freshwater will become more scarce and water resource manager predict that water rights and water management issues will become the most common reason for legal battles and military conflicts among developing nations.

There are costs associated with strategies for addressing global warming, and there are costs associated with failing to address global warming. It seems likely that the costs of failing to address global warming will be far greater in the long run (say by 2100). So any investments humans make today to slow the pace of global warming will pay dividends in the long run by helping to mitigate the problem while keeping it more economically manageable.

Since greenhouse gases are long-lived, the planet will continue to warm for the next several decades even if we stop or reduce emissions today. But the degree to which global warming will happen and change life on Earth depends upon our decisions today.

How will the Earth system change in the future?
As the world consumes ever more fossil fuel energy, greenhouse gas concentrations will continue to rise and Earth's average temperature will rise with them. The Intergovernmental Panel on Climate Change (or IPCC) estimates that Earth's average surface temperature could rise between 2°C and 6°C by the end of the 21st century.

For most places, global warming will result in more hot days and fewer cool days, with the greatest warming happening over land. Longer, more intense heat waves will happen more often. High latitudes and generally wet places will tend to receive more rainfall, while tropical regions and generally dry places will probably receive less rain. Increases in rainfall will come in the form of bigger, wetter storms, rather than in the form of more rainy days. In between those larger storms will be longer periods of light or no rain, so the frequency and severity of drought will increase. Hurricanes will likely increase in intensity due to warmer ocean surface temperatures. So one of the most obvious impacts of global warming will be changes in both average and extreme temperature and precipitation events.

Scientists are also monitoring the great ice sheets on Greenland and West Antarctica, both of which are experiencing increasing melting trends as surface temperatures are rising faster in those parts of the world than anywhere else. Each of those ice sheets contains enough water to raise sea level by 5 meters and if our world continues to warm at the rate it is today then it is a question of when, not if, those ice sheets will collapse. Some scientists warn we could lose either, or both, of them as soon as the year 2100.

Ecosystems will shift as those plants and animals that adapt the quickest will move into new areas to compete with the currently established species. Those species that cannot adapt quickly enough will face extinction. Scientists note with increasing concern the 21st century could see one of the greatest periods of mass extinction of species in Earth's entire history. Ultimately, global warming will impact life on Earth in many ways. But the extent of the change is up to us.