Climate change is simultaneously one of the most contentious and ubiquitous issues of our time. It seems like every time we turn on the TV or read the newspaper, there's something new being said about climate change. And depending on who's talking, it can be presented as anything from the end of the world as we know it to a hoax invented by venal scientists pushing a socialist agenda.

At its heart, however, climate change is about science. Our understanding of the earth's climate system is built on scientific knowledge derived from the fields of electromagnetic radiation, non-linear fluid dynamics, physical chemistry, ecology, and many more. If this is indeed the case, then it begs the question: can science help us distinguish fact from fiction?

I have a unique chance--even a mandate--to tackle this question as an atmospheric scientist and director of a climate science center at a public university. I also have a personal stake in this issue, and it's not the research dollars. Our friends, our neighbors, even our family members often ask about it as well. Accepting a consensus view just because "they" say it is so is not an option for me.

I have both the opportunity and the responsibility to analyze the data myself and make up my own mind. Is our climate really changing? If so, then what is causing this change? And, why should I care?

Here is what I've found.

Is our climate changing?

Temperature records from weather stations around the world are the most obvious place to look for information on whether climate is changing. Many of these individual records are available free of charge from the Global Historical Climatology Network.

What we really care about, however, is not what's happening in one day or at one location, or even over one decade or in one country. Climate is defined as the average weather conditions over 20-30 years or more, and the question here is whether the average temperature of the planet is changing or not. For this reason, four independent research teams around the world have undertaken the Herculean task of homogenizing and averaging all of these individual records into a global average: NASA, NOAA, the UK Met Office, and the Japanese Meteorological Agency.

A fifth independent global temperature analysis was published recently. Although it only includes land-based temperature records, this analysis also identified a long-term warming trend consistent with previous analyses. This record is particularly compelling as it was assembled by a team led by Dr. Richard Muller, a physicist from UC Berkeley who questions the scientific consensus on global warming.

Thermometer-based records are clear: temperatures over the last 160 years have increased. Over climate timescales, in both rural and urban locations, over the land and the ocean, in the northern hemisphere and south, the planet is warming.

Of course, thermometer-based records are recorded by humans and are therefore subject to error. But a host of natural indicators tell the same story. When observed changes in over 29,500 natural indicators--including the width and density of tree rings, the date of plants flowering, the northward migration of cold-intolerant pests, and others, many of which can be observed in our own backyards--were compiled and analyzed, over 90% had trends consistent with increasing temperatures. These records provide a secondary and independent confirmation that the planet is indeed experiencing a warming trend.

What is causing this change?

We know that natural forces, like the sun, or natural cycles, have changed our climate in the past and are likely to change it again in the future. We also know that we can't control these forces. If we want to understand why the earth's average temperature might be changing, we need to start with these natural factors, not human ones. We need to determine if these "natural suspects," so to speak, are to blame for what is happening to our climate-just as they have been before.

Natural cycles like El Niño--and other lesser-known cycles, such as the Atlantic Multi-Decadal Oscillation--are all part of the internal variability of the earth system. Because these cycles are internal to the climate system, they cannot spontaneously generate or destroy heat: that would violate the fundamental physical principle of conservation of energy. Rather, these natural cycles affect the earth's temperature by redistributing heat around the world. During a La Niña episode, for example, more heat is transferred from the atmosphere into the ocean, so global average temperature is usually a little cooler than average. In contrast, during an El Niño event, more heat is being transferred from the ocean into the atmosphere, so global temperature is usually a little warmer than average.

If natural cycles were responsible for the current warming, they could only be accomplishing this by moving heat from the ocean into the atmosphere. The heat content of the atmosphere should have increased while the heat content of the ocean should have decreased (but only very slightly, because the ocean's heat capacity is so much greater than the atmosphere's).

Observations, however, paint a very different picture. Over the last 50 years, the heat content of the ocean, the atmosphere, and the land has increased. Not only that, but the increase in ocean heat content is twenty times greater than the increase in the heat content of the atmosphere, land surface, and cryosphere (ice) combined.

If the oceans, atmosphere, land and cryosphere are all warming, natural cycles internal to, or inside, the earth's climate system cannot be causing the current warming. No, the warming must be coming from an external force, and there are only two very important natural external forces left to suspect: geologic activity, including volcanoes (external in the sense that they are not part of the atmosphere, biosphere, or ocean) and the sun.

Volcanoes can affect climate in two ways. When they erupt, volcanoes spew enormous amounts of dust and ash into the atmosphere. If the volcano is powerful enough, like Pinatubo in 1992 or Krakatua in 1883, the dust and particles can reach all the way into the stratosphere where they can circle the globe for weeks and even years. Volcanic particles reflect the sun's energy back to space, cooling (but never warming) the earth.

In contrast, long-term geologic activity (including both volcanic eruptions and, more importantly, slow seepage from the ground in active geologic areas) releases carbon dioxide and methane from deep within the earth's crust. Both carbon dioxide and methane are powerful heat-trapping gases. Detailed measurements have showed that volcanic and other geologic activity produces an average of 0.13 to 0.44 gigatons of carbon dioxide and 53 teragrams of methane per year. However, those amounts only add up to around 1% of the carbon dioxide and less than 15% of the methane that humans are producing per year. So while long-term geologic activity is very likely contributing to a warming of the planet through releasing heat-trapping gases from the earth's crust into the atmosphere, this contribution is much smaller than the amounts of the same gases currently being produced by human activities.

The second potential external cause of warming is the sun. The sun is the most important natural control on climate. When the sun gives off more energy, the earth warms; when it gives off less, it cools. Over longer periods of time, periodic shifts in the earth's orbit (the same shifts that have gradually unaligned the pyramids from the stars they were originally designed to track) can also affect how sunlight falls on the earth, and through that, the earth's temperature. So could either of these be responsible for our current warming?

Records of the sun's energy show an 11-year repeating cycle, correlated with sunspots and solar activity. They also show a long-term upward (warming) trend until the mid-1970s, followed by a slow downward (cooling) trend since then.

Correlation doesn't necessarily imply causation: but lack of correlation does eliminate the possibility of causation. Over the first part of the century, increasing energy from the sun had a positive but likely relatively small effect on temperature. But in the last 30 years or more, if our earth's temperature were responding to changes in the sun's energy, it should have been cooling, not warming.

In terms of the periodic shifts in the earth's orbit, it turns out that the predictions of global cooling back in the 1970s were not that far off base. In the natural course of things, the next item on the earth's agenda is a return to a glacial maximum, or what we often call an ice age. Careful analysis suggests this would happen sometime in the next 1500 years (which may seem long to us, but in geologic terms is no more than the blink of an eye). So the sun and the earth's orbit both have a perfect alibi: neither can be blamed for our current warming.

If natural cycles, volcanoes, and the sun can't be causing the warming, then and only then do we have a reason to start to look for a new factor that might affect climate. And it turns out that scientists have been aware of such a factor for quite some time.

In 1824, Joseph Fourier established the importance of natural levels of heat-trapping gases in the atmosphere, including water vapor, carbon dioxide, and methane, to maintaining the earth's temperature. A few decades later, John Tyndall showed how these gases are nearly transparent to incoming short-wave energy from the sun, but opaque to outgoing long wave, or heat energy, from the earth. The gases absorb the earth's heat that would otherwise escape to space, keeping the earth's temperature nearly 60^oF warmer than it would be otherwise. This is the basis of the natural greenhouse effect.

By the late 1800s, scientists had also realized that wide-spread burning of coal, oil, and natural gas was increasing levels of carbon dioxide in the atmosphere by releasing carbon that would otherwise remain trapped deep below ground. An unexpected side effect of the Industrial Revolution was an enhancement of the natural greenhouse effect. What was not yet clear was the degree to which this would affect the temperature of the planet.

In 1890 Svante Arrhenius became the first to calculate the effect of burning coal, oil, and natural gas on global temperature. The results of this first "climate model," based on fundamental physics and chemistry and computed entirely by hand, confirmed that human emissions of carbon dioxide and other heat-trapping gases could be enough to significantly alter the energy balance of the earth. Thousands of studies since then have continued to estimate and document the effect of rising carbon dioxide levels on the earth's temperature.

To summarize: for natural cycles, the sun, or volcanoes to be driving the recent warming of the earth's climate would violate the fundamental laws of physics and no one, to date, has explained how this could be the case. Other plausible explanations, such as cosmic rays, have proven inadequate to explain both the magnitude of the warming as well as to explain how increasing levels of heat-trapping gases in the atmosphere are not warming the planet.

On the other hand, an artificial enhancement of the natural greenhouse effect as a result of increasing emissions of heat-trapping gases from human activities is entirely compatible with the fundamental laws of physics, and can explain the recent warming requiring no more than basic physics and chemistry that are entirely compatible with the fundamental laws of physics. The majority of science to date supports the theory of a human-induced change in global climate.

Why should we care?

Even if climate is changing, and humans are contributing to this change, why worry? Won't a warmer climate have many beneficial effects for northern countries? Or won't it be cheaper to just adapt to future change than revolutionize our energy industry now?

Being from Canada, I know that a longer growing season is a good thing. Warmer weather is, too. Less frequent and less severe cold spells can save lives. And increasing levels of carbon dioxide can be beneficial to some types of plants (not all). There's no question that a warmer climate could bring benefits, particularly for northern countries.

At the same time, however, warmer weather means more frequent and extreme heat waves that also take a toll on human health. Many of the world's largest cities lie within a few feet of sea level. As the oceans heat up and expand, the risk of coastal flooding increases. Warmer air means faster evaporation, higher humidity levels, and greater risk of heavy precipitation and flooding across many mid-latitude regions, including the Midwest and Northeast U.S. Areas already water-short could face increasing pressure, as more water would be required to provide the same amount of irrigation under warmer temperatures. Regions like the U.S. West, where much of the water comes from snowmelt, would have to invest in large dams and other infrastructure to store the winter rains that previously fell and were naturally stored as snow.

These types of pluses and minuses are exactly what we found in two of our recent studies. Our first recent study showed that while climate change is likely to increase risk of fire in mid-latitudes (due to warmer temperatures and shifting precipitation patterns), it could decrease risk in some parts of the tropics (due to increasing precipitation). The second study showed that while climate change could increase risk of dengue outbreaks in a northern U.S. city (as the weather becomes warm enough for the disease to incubate in the mosquito), it could actually slightly decrease the risk in southern cities (because it would be too hot, the mosquito wouldn't live long enough to incubate the disease).

At the same time, however, these two studies and a host of others clearly show how the risks of a changing climate generally outweigh the benefits. This is because human society-our buildings and infrastructure, our energy, even our health--is perfectly adapted to our current climate. Any change, whether a cooling or a warming, will have a negative effect because we will need to adapt in response, and adaption (for any reason) is usually more expensive than maintaining the status quo.

Taking action to reduce the impact humans are having on climate does not require a draconian response, however, or a lifestyle of deprivation and hardship. The most important thing we can do to address climate change is to transition in a sensible way from the limited resources of fossil fuels to the infinite abundance of renewable energy. And by doing that, even if we were wrong about climate change, we would end up with a society powered by energy that does not pollute our air and our water; that is sourced and maintained by our local communities; that does not rely on foreign resources; and that will never run out.

In that sense,

And that is a very different question.

There are many other interesting questions on climate change that I could not address here: from claims that global warming has slowed or stopped to challenges to the "hockey stick" analysis of temperature over the last 2,000 years. These and many more are thoroughly discussed at the websites RealClimate.org and SkepticalScience.com, which provide links to original datasets and journal articles. I highly recommend these resources to anyone interested in further investigation of the data and evidence for climate change.

Katharine Hayhoe is an atmospheric scientist and director of the Climate Science Center at Texas Tech University. With her husband Andrew Farley, she wrote A Climate for Change: Global Warming Facts for Faith-Based Decisions, a book that untangles the complex science and confronts many of the most common questions on climate change.