Energy from the sun drives the Earth’s weather and climate. Energy from the sun heats the Earth’s surface and in turn, the Earth radiates energy back into space. Naturally occurring greenhouse gasses (such as water vapor, carbon dioxide and methane) trap some of the outgoing energy (or radiation). These gasses serve an important role in sustaining life on Earth by retaining heat somewhat like the glass panels of a greenhouse. Without this natural “greenhouse effect,” temperatures would be much lower than they are now and life as we know it today would not be possible. Instead, thanks to greenhouse gasses, the Earth’s average temperature is a very hospitable 60 degrees F.

As mentioned, carbon dioxide is a naturally occurring greenhouse gas and like water, is constantly cycled. Life on earth is carbon-based, and the source of that carbon is atmospheric carbon dioxide. Carbon dioxide is naturally produced by the respiration of animals, by the decomposition of plants, and by forest fires as well as volcanic eruptions. All the while, the oceans absorb carbon dioxide from the atmosphere and plants consume carbon dioxide in the process of photosynthesis.

Despite the natural occurrence of carbon dioxide and other greenhouse gasses, it is clear that certain human activities have added to the concentration of greenhouse gasses in our atmosphere. Increasing levels of carbon dioxide and methane in the atmosphere have been well documented in recent years. According to the EPA (the Environmental Protection Agency), since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have gone up nearly 30% while methane concentrations have more than doubled. These increases are magnifying Earth’s natural greenhouse effect, but to what extent?

A warming trend of about 1 degree F has been recorded since the late 19th century. Warming has been noted in both the northern and southern hemispheres. Melting glaciers and a decreased snow pack in many areas are feeding speculation that global warming is real and accelerating. In fact, scientists believe that sea level has risen by about 6 inches over the past century globally. However, is this just a part of a natural cycle or are we actually seeing the start of a large scale, human induced climate change?

Figuring out to what extent human activities have altered the chemical composition of the atmosphere is not easy. This is because many factors, both natural and human, affect our planet’s temperature such as changing amounts of incoming solar radiation and natural climatic variation. Current scientific understanding of these other factors remains incomplete. In short, scientists think rising levels of greenhouse gasses are contributing to global warming, as would be expected. But to what extent is difficult to determine at the present time. As atmospheric levels of greenhouse gasses continue to rise, scientists estimate average global temperatures will continue to rise as a result, but by how much and how fast?

As already mentioned, many greenhouse gasses are naturally occurring. One major reason for these naturally occurring greenhouse gasses are volcanoes. There are currently 70 active volcanoes in the United States alone. These quaking mountains constantly emit large levels of water vapor, carbon dioxide and methane into the atmosphere. However, looking back through history you will find that following large volcanic eruptions, notable world wide cooling has taken place. For instance, the eruption of Mt. Pinatubo (Philippines) in 1991 produced an average drop in world temperature by nearly 2 degrees F in the subsequent two years. The reason for the drop in temperature was not because of the vast amounts of greenhouse gasses that were emitted during the eruption, but because of the sulfates and particulate matter (ash) that were also ejected into the atmosphere. Sulfates and other particulate matter act to reflect incoming solar radiation leading to less heating at the Earth’s surface.

The eruption of Mt. Tambora (Indonesia) in 1815 resulted in an extremely cold spring and summer in 1816, which became known as the year without a summer. The Tambora eruption is believed to be the largest volcanic eruption in the last ten thousand years producing massive amounts of both greenhouse gasses as well as particulate matter. North America and Europe were hit exceptionally hard. Snow occurred in New England and the Great Lakes during the months of June, July and August. Sea ice migrated across Atlantic shipping lanes and alpine glaciers advanced down mountain slope to exceptionally low elevations.

Although volcanic eruptions don’t produce as much greenhouse gas as man does on an annual basis, it has been shown that sulfates and particulate matter may have a larger impact on planetary temperature. In addition to the constant release of greenhouse gasses, humans are also releasing large amounts of atmospheric particles, such as sulfates, into the atmosphere. Since these pollutants are also increasing in time, scientists expect sulfates to at least partially offset the global warming due to greenhouse gasses.

If atmospheric levels of greenhouse gasses continue to rise at the current rate, scientists estimate that average global temperatures will continue to rise as a result. The Intergovernmental Panel on Climate Change (IPCC) projects further global warming of anywhere between 2 and 10 degrees F will be possible by the year 2100. The very large range is a result of the many uncertainties such as; greenhouse gas emissions, solar irradiance, the possible cooling effects of atmospheric particles such as sulfates and the climate’s response to changes in the atmosphere. The IPCC states that even the low end of this warming projection “would probably be greater than any seen in the last 10,000 years but the actual annual to decadal changes would include considerable natural variability.”

Although many scientist are fairly confident about there projections on global temperature and precipitation change, they are less confident about the ones for small-scale areas like local temperatures and precipitation patterns. This is largely due to the fact that current computer models used to forecast global climate change are not finite enough to accurately simulate how things may change at smaller scales.

The IPCC cautions, “Complex systems, such as the climate, can respond in non-linear ways and produce surprises.” One common belief is that a warmer Earth will produce more frequent and intense storms including hurricanes. Recent history will certainly support this claim as 2005 provided an unprecedented number of Atlantic hurricanes as well as some of the strongest hurricanes recorded this century. Although we appear to be going into a new era of heightened hurricane activity, this cannot be attributed to global warming alone.

Scientists believe that the recent increase in hurricane activity is largely a result of naturally occurring long-term fluctuations in sea surface temperature in the Atlantic, known as the Atlantic multidecadal oscillation (AMO). The AMO can cause warmer than average sea surface temperatures in the Atlantic over long periods of time. Scientists believe the last time this occurred was in the 1950’s and 60’s. It was also during that period that the Atlantic was experiencing frequent and powerful hurricanes such as Beulah in 1967 and Camille in 1969. The 1970’s and 80’s brought a dramatic decrease in hurricane activity in the Atlantic which corresponded to cooler than average water temperatures.

Since 1995, however, the Atlantic has warmed once again resulting in the recent active hurricane seasons. Many scientists argue, however, that global warming may be increasing the effect of the AMO causing the rash of hurricane activity over the past decade. Unfortunately, the current warm phase of the AMO is expected to last at least another decade and could very well produce even stronger storms as sea surface temperatures continue to rise.

The current state of global warming research can’t provide definitive answers to all of our questions. It is a certainty that human activities are adding greenhouse gasses to our atmosphere and that these gasses tend to warm our planet. The big questions such as: How much warming will occur? How fast will this warming occur? And how will the climate respond to this warming? Remain unanswered and will likely remain that way for several more decades.