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Climate Change

Human Activity Far More Responsible For Rising Seas Since The Mid 20th Century

Article:

Slangen, A.B.A., Church, J.A., Agosta, C., Fettweis, X., Marzeion, B., Richter, K., 2016, Anthropogenic forcing dominates global mean sea-level rise since 1970, Nature Clim. Change Advanced Online Publication, 1-7, doi: 10.1038/nclimate2991.

 

Background:

It should come as no surprise that global sea level has risen over the 20th century. In fact, observational data points to a global rise of about 7 inches between 1900 and 2005. An important question remains: what is causing sea level to rise? The major contributors to sea level rise are well known and understood: Thermal expansion of the oceans and glacier mass loss.

Thermal expansion is a physical property of water. As the oceans warm, molecules of water move slightly further apart. This change, when summed across the world’s oceans, causes the water in the oceans to take up more physical space. This translates to a rise in sea level. Glacier mass loss is the melting of glaciers on land due to increased planetary warmth. The meltwater from the glaciers enters the oceans and causes sea level to rise.

But what is driving these contributions to sea level rise? Everything being held constant, glaciers just don’t melt and oceans don’t expand without some type of coercion!

To best address this question, scientists led by Aimee Slangen from CSIRO Oceans & Atmosphere, looked at the natural and anthropogenic (human-caused) climate influences on global sea level change.

 

Methods:

Scientists went to work using climate models to investigate natural and human caused climate influences on sea level rise over the 20th century. In a series of experiments, Slangen and her team quantified the contributions of green house gases, aerosols produced by human activities, natural changes in incoming sunlight and historical climate variability to sea level rise, and compared these results with the observed global rise in sea level. The model results were then analyzed to determine the influence of natural and anthropogenic influences on sea level rise.

 

Results:

Prior to 1950, natural climate processes contributed to about 70% of sea level rise. In strong contrast, from 1970 onwards, nearly 70% of sea level rise was purely in response to human activities. In other words, following the year 1970, human activities are mostly to blame for sea level rise (Figure 1).

oceanbites29_fig1

Figure 1. Comparison of influencing factors on global sea level rise during the 20th century. Human activities (top panel) had a much strong influence on sea level rise after 1970, whereas natural climate factors (bottom panel) influenced sea level rise more strongly prior to 1950.  Adapted from Slangen et al., 2016 and climate central.org

Unsurprisingly, the influence of natural climate change on sea level rise over the entire 20th century cancels itself out, with almost no meaningful contribution to sea level rise. So what was happening with the natural climate system, causing it to have such strong influence on sea level before 1950? During the first half of the 20th century, sea level was still feeling the influence of the Little Ice Age, a naturally cool period of time between ~1300 and 1900. Sea level naturally rose more quickly as the planet came out of this cool period, slowing as sea level became more representative of the warmer climate.

As the natural climate influence began to dwindle, human activities such as increased greenhouse gas emissions and reductions in sun reflecting aerosols (soot and dust) began to dominate the influence on sea level rise, becoming even more pronounced after 1970.

 

Significance:

Global sea level change has been in the spot light of climate studies, and for good reason. Rising sea levels are a major threat to coastal communities (read: many of the most populous cities in the US and the world). With rising sea level comes a whole host challenges including increasingly frequent coastal flooding and destructive erosion. As a global community, we need to understand how human activity is contributing to sea level rise. This includes an understanding of how much damage has been done, in addition to accurate projections of future sea levels so that we can best prepare our coastal communities.

 

Discussion

3 Responses to “Human Activity Far More Responsible For Rising Seas Since The Mid 20th Century”

  1. This article was very informative and we really liked it. To us the most interesting part about the article was where you explained Aimee Slangen’s results. After she ran a series of experiments, she found that before the 1950s, natural climate processes added up to more than 70% of sea level rising. In shocking contrast, after the 1970s, virtually all of sea level rising was attribute to human activity.

    Posted by Rita Manavalan and Jasmine Kumar | May 24, 2016, 1:43 pm
    • Also what experiments did Aimee Slangen do to get these results?

      Posted by Rita Manavalan and Jasmine Kumar | May 24, 2016, 1:45 pm
      • Thank you for your reply, and a very good question! Aimee Slangen and her colleagues used a series of climate models for their experiments. Climate models are math based computer simulations that help climate scientists learn about past, present and future climates. These models rely on good historic data such as air and sea temperatures, the amount of carbon dioxide in the atmosphere and many other variables. For these experiments, the scientists used climate models that relied on data from the CMIP5 (Coupled Model Intercomparison Project Phase 5) to learn about the contributions of greenhouse gases and natural climate change to twentieth century sea level rise.

        Posted by Brian Caccioppoli | May 26, 2016, 11:05 am

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