The scientific elements of climate change are well-established:

• Greenhouse Effect. When energy from the sun enters the atmosphere, some of it warms the earth’s surface while the excess bounces back into space. Greenhouse gases, however, trap this energy. Thus, they warm up the earth.
  
  As a natural process, this is not bad. If GHGs did not have this capacity, then the earth’s average temperature could never have warmed up enough to support life.
  
  The balance of gases in the atmosphere is critical to maintaining the biosphere in a relatively stable, resilient state, and maintaining the natural ebb and flow of the carbon cycle. However, recent human activities have thrown these natural cycles out of balance. Now there are too many GHGs in the atmosphere, and they are holding too much heat. CO2 is especially a problem. 
  
  
• Drivers of climate change. Many powerful forces play into the dynamic of climate change. 
  

  Nature. Over the eons, natural factors - volcanic eruptions, solar activity, meteorites, tilts in the earth’s orbit, changes in ocean circulation, etc. - have caused the earth to warm and cool. Sometimes these forces work together, creating feedback loops that intensify or dampen the others’ effect.  Sometimes, one force alone can spin the whole system into radical change.
  
  Climate can shift very abruptly, but it can also remain stable for long periods of time. Agriculture (and human civilization as we know it today) has evolved during the last 10,000 years since the last major Ice Age, a period of relative climate stability.
  
  Humans. Beyond any other species, human have an impact on the climate – especially through the production of GHGs. Particularly in the developed world, these emissions come primarily from a dependence on fossil fuels for electricity generation and transportation. Throughout the globe, GHG emissions also result from destructive land use patterns such as deforestation, urbanization, erosion and excessive soil disturbance, etc.
  

• Evidence. Climate scientists have gathered this data from many different sources. Climate (rather than weather) must be studied over broad scales of time. 
  

  Paleoclimatology. This is the study of the earth’s previous climates, based on samples taken from ice cores, ocean sediments, tree rings, coral reefs, etc. These sources provide miniature historical records, revealing the amounts of greenhouse gases in the atmosphere, extended periods of heating and cooling, and other crucial information.
  
  As paleoclimatologists understand how forces of climate change worked in the past, they then apply these principles to the present and future. Climatologists also work from temperature records kept for the past hundred or so years.
  
  Recent Climate Findings. Since pre-industrial times, carbon dioxide in the atmosphere has increased 31%, and atmospheric methane has increased 151%. During the 20th century, the average global temperature increased about 1°F.
  
  Snow cover, sea ice, and glaciers began to melt significantly, sea level and temperature rose, and precipitation amounts and severity increased. Near the end of the century, average minimum and maximum daily temperatures rose.
  
  Climate Models. Climate models are complex computer simulations that project (not predict) future climate shifts based on an enormous range of factors – energy transfers, land use patterns, fluctuations in atmosphere, polar ice, oceans, and land mass, precipitation patterns, carbon sinks, etc.
  
  Current models estimate that the global temperature could rise 4-7 degrees Fahrenheit during the 21st century. Even a 2 degree temperature rise will significantly affect the climate.
  
  Past climate models have overestimated some details, while underestimated others – for example, none of them projected the record Artic polar ice melt that occurred in the summer 2007. Nor did they anticipate the ocean’s apparently lessening capacity to serve as a carbon sink.
  
  Standards of Certainty. By its nature, science evolves. Scientists remain committed to the spirit of inquiry, always seeking new data and interpretations. As a result, certainty is usually not part of their standard vocabulary.
  
  In contrast, the 2007 IPCC report on climate change used unequivocal language – employing criteria such as “virtually certain” (greater than 99% chance), “extremely likely” (better than 95%), and “very likely” (greater than 90%) - to describe the likelihood that climate change is happening at an unprecedented rate, and that humans are a big part of the problem.
  
  There is more than enough evidence for us to prepare – quickly – to manage the risks of greenhouse gas emissions. 
  

• Remaining Questions. Scientists are not sure how much the climate will change, how fast or how slow the changes will happen, or what the exact effects will be.
  
  Areas for more study. Scientists are working on developing better regional and local climate models. Also, they seek to better understand the mechanisms of abrupt (versus gradual) climate change.