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Are humans really causing climate change?


  • How do scientists know human activities are the cause?


Answer:

Prior to the Industrial Revolution and subsequent exploding world population, the Earth’s climate changed in response to natural forcings.  These changes to the climate generally evolved over long periods of time, shifting from ice age to interglacial period and back again.

Since industrialization, the human hand in forcing the climate has become evident.  Current global warming trends are not able to be explained by natural forcing alone.  Anthropogenic (human) sources of warming need to be factored in if we are to fully explain the trends that we are currently observing:

Click figure to enlarge

Figure 1: Comparison of observed changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings.


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Atmospheric carbon dioxide changes


Carbon dioxide (and other greenhouse gases) is added to the atmosphere by natural processes and by human activities.  In the past, carbon dioxide levels have been able to remain fairly constant as the release was in balance with the uptake.  Today, humans are releasing carbon dioxide into the atmosphere at an accelerated rate.  This increased contribution is occurring too quickly for the environment (the oceans, vegetation) to respond and absorb the excess.  As a result, carbon dioxide concentrations in the atmosphere are increasing.

The main sources of the excess carbon dioxide can be traced back to humans.  Since the Industrial Revolution, we have relied on fossil fuels to power our lives.  We use fossil fuels to get around, to heat our homes, and to drive our industries.  As our dependence on fossil fuels grows, so do our carbon dioxide emissions.


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Land use


Humans are not only releasing excess carbon dioxide to the atmosphere, but are changing the planet’s ability to deal with this excess.  As the world’s population grows, we take over greater portions of our planet’s surface.  We clear land to make way for cities and human settlement.  We manipulate our landscape in order to grow more food, and allow more livestock to graze.  By altering the natural landscape, we are altering the environment’s ability to deal with the excesses that we have created.