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Why it is believed that climate change is mainly due to mankind’s activities?

Can you please explain to me why it is believed that climate change is mainly due to mankind’s activities, rather than the normal ebb and flow of change that has occurred since the earth was formed? I’m not a CC denier, just need to evidence to persuade a particularly obdurate work colleague that we have to change our ways!
Peter Bowden from Essex (Age 55+)

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4 Responses

  1. the following URLs may be of some use and contain some very useful, and perhaps more importantly, reliable information…
    http://environment.newscientist.com/channel/earth/dn11462
    http://news.bbc.co.uk/1/hi/in_depth/629/629/7074601.stm
    http://www.metoffice.gov.uk/corporate/pressoffice/myths/index.html

  2. The Earth’s climate has indeed fluctuated from extreme to extreme over it’s 4.6 billion year existence, for various natural reasons, but mostly the transition periods were millions of years.
    Excepting the beginning of the current glacial period, humans weren’t around to experience those changes but the geological record shows that mass extinctions coincided with many climate change events.
    The current warming certainly cannot be solely attributed to anthropogenic activities, but we have undoubtedly exacerbated the situation, thereby speeding up a process that usually takes millions of years.
    Disasters on a global scale will be the result of rapid climate change, because we are simply not prepared for it, which is why we need to do everything we can to slow down the process by minimizing any further increase in atmospheric CO2 levels.

    Emily Sear, Geosciences Undergraduate, The Open University

  3. Burning fossil fuels at large scale started with the industrial revolution. If you look at the historical carbon-dioxide records from ice cores at Low Dome, East Antarctica by Etheridge et al. (see http://cdiac.ornl.gov/trends/co2/lawdome.html), they unambiguously show a considerable increase in carbon-dioxide that coincides with the end of XVIII century. This is definitely not a normal geological time-scale phenomenon. Direct link between the increase of carbon-dioxide concentration in atmosphere and climate change is difficult to establish. However, a wait and see strategy when it comes to recognising the climate change will not get us very far.

  4. At the root of our understanding of human-induced climate change is some basic science, which relies on fundamental physics rather than observations of climate or climate models. Observations and climate modelling studies have supported predictions based on the underlying science.

    The basic physics in question is that certain gases such as carbon dioxide (CO2) absorb heat. So, if we increase the amount of CO2 in the atmosphere, we expect more heat to be absorbed, and the atmosphere to warm. Now, we know that we have put a lot of CO2 into the atmosphere by burning fossil fuels, but also as a result of agriculture and land use change. We have also measured atmospheric CO2 concentrations and have seen that they are rising. At the same time we have observed that global temperatures are increasing at the same time. Of course it’s not as simple as that – there have been lots of studies that have worked on quantifying the amount of heating due to increases in CO2 and other greenhouse gases (GHGs), on attributing warming to elevated GHG concentrations by examining whether observed changes are outside the range of expected natural variability, and so on.

    So, at base we have some very basic and very well understood physics which tells us to expect manmade global warming. On top of that we have thousands of studies examining the details of the warming and how this relates to increases in GHG concentrations and natural variability.

    Unless we take pretty immediate action to cut GHG emissions by a huge amount, it’s likely that the world will warm by at least 3 degrees C on average sometime in the second half of the 21st century. The last time the Earth was this warm was about 3 million years ago, when sea levels were some 15-20 m higher than they are today, and when there is some evidence of more-or-less permanent El Nino conditions. Without action to reduce emissions the end of the 21st century is likely to see a temperature rise of more than 3 degrees C – high-end projections suggest a 4-6 degree rise is possible. This is comparable to the difference between an ice age and a warm interglacial, but higher than anything the Earth has experienced for many tens of millions of years.

    More on the basic science of global warming can be found at http://www.realclimate.org. An article dealing with the basic physics can be found at http://www.realclimate.org/index.php/archives/2007/12/les-chevaliers-de-l%e2%80%99ordre-de-la-terre-plate-part-ii-courtillots-geomagnetic-excursion/#more-504.

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