In recent years, global warming and climate change have been the subject of a great deal of political controversy, especially in the U.S. But as the science becomes clearer and consensus grows impossible to ignore, debate is moving away from whether humans are causing warming and toward questions about how best to respond.

Temperatures rising

View Images Global Land-Ocean Temperature Index Chart courtesy NASA/GISS

Evidence of rising temperatures is pervasive and striking: Thermometer records kept over the past century and a half show Earth's average temperature has risen more than 1 degree Fahrenheit (0.9 degrees Celsius), and about twice that in parts of the Arctic.

That doesn’t mean temperatures haven't fluctuated among regions of the globe or between seasons and times of day. But by analyzing average temperatures all over the world, scientists have demonstrated an unmistakable upward trend.

This trend is part of climate change, which many people consider synonymous with global warming. Scientists prefer to use “climate change” when describing the complex shifts now affecting our planet’s weather and climate systems. Climate change encompasses not only rising average temperatures but also extreme weather events, shifting wildlife populations and habitats, rising seas, and a range of other impacts.

All of these changes are emerging as humans continue to add heat-trapping greenhouse gases to the atmosphere.

How is climate change measured?

Although we can't look at thermometers going back thousands of years, we do have other records that help us figure out what temperatures were like in the distant past. For example, trees store information about the climate in the place they’re rooted. Each year trees grow thicker and form new rings. In warmer and wetter years, the rings are thicker. Old trees and wood can tell us about conditions hundreds or even thousands of years ago.

Windows on the past are also buried in lakes and oceans. Pollen, particles, and dead creatures fall to the bottom of oceans and lakes each year, forming sediments. Sediments contain a wealth of information about what was in the air and water when they fell. Scientists reveal this record by inserting hollow tubes into the mud to collect layers of sediment going back millions of years.

For a direct look at the atmosphere of the past, scientists drill cores through the Earth's polar ice sheets. Tiny bubbles trapped in the ice are actually samples from the Earth's past atmosphere, frozen in time. That's how we know that the concentrations of greenhouse gases since the Industrial Revolution are higher than they've been for hundreds of thousands of years.

Computer models help scientists to understand the Earth's climate, or long-term weather patterns. Models also allow scientists to make predictions about the future climate by simulating how the atmosphere and oceans absorb energy from the sun and transport it around the globe.

We are the reason

Several factors affect how much of the sun's energy reaches Earth's surface and how much of that energy gets absorbed. Those include greenhouse gases, particles in the atmosphere (from volcanic eruptions, for example), and changes in energy coming from the sun itself.

Climate models are designed to take such factors into account. For example, models have found that changes in solar irradiance and volcanic aerosols have contributed only about two percent of the recent warming effect over 250 years. The balance comes from greenhouse gases and other human-caused factors, such as land-use changes.

The speed and duration of this recent warming is remarkable as well. Volcanic eruptions, as an example, emit particles that temporarily cool the Earth's surface. But they have no lasting effect beyond a few years. Events like El Niño also work on fairly short and predictable cycles. On the other hand, the types of global temperature fluctuations that have contributed to ice ages occur on cycles of hundreds of thousands of years.

The answer to the question, “Is global warming real?” is yes: Nothing other than the rapid rise of greenhouse gas emissions from human activity can fully explain the dramatic and relatively recent rise in global average temperatures.