I ended my last post with a startling depiction of the effect of global warming on our oceans, revealing that they are absorbing 93.4% of the heat content from global warming. This was calculated from IPCC data for the period 1993-2003 based on
the energy content changes in different components of the earth system (see
figure 1 below).
Figure 1: Energy content changes in different components of the Earth system for the two periods 1961-2003 (lilac) and 1993-2003 (pink). (IPCC AR4 5.2.2.3)
Mention the words ‘climate change’ and most
people will immediately think of the greenhouse effect and depleting ozone
layer as a result of human activities such as burning fossil fuels but this
evidence makes it clear we need to be more aware of the changes occurring in
our oceans.
If this isn’t enough to convince you, the
paramount importance of this colourless liquid becomes clear when considering
the following:
- Water covers just over 70% of the Earth's surface.
- Water provides 99% of available
living space on the planet.
- 80% of all life on Earth
depends on healthy oceans and coasts.
- More than a third of the
world’s population lives in coastal areas or on small islands, despite only accounting for less than 4% of the Earth’s land.
And new research (Halpern et al. 2015) has shown the extent to
which humans are affecting the oceans. They investigated changes in cumulative
human impacts using 19 different anthropogenic stressors on the world’s oceans over a 5-year period (2008-13). Their findings indicated that ‘no part of the
global ocean is without human influence’ and that almost the whole ocean
(97.7%) is affected by multiple stressors. Over the 5-year period,
approximately 66% of the ocean experienced an increase in cumulative impact and
in addition, there are several worrying ‘hotspots’ most notably in the North
Sea and the South and East China Seas where nearly all of the stressors overlap.
Although there were decreases in all stressors in some areas, these were
relatively small and limited in area and in general, the cumulative impact
across all stressors is increasing. Unsurprisingly, coastal areas are
experiencing the greatest increases.
The study also showed that those stressors
associated with climate change (ocean acidification, high sea surface
temperatures and increasing UV radiation) are dominant but that other human activities
such as commercial fishing and shipping also play a significant role.
The contribution of each of the human stressors to overall impact in 80 of the most affected countries (Halpern et al. 2015)
The world’s oceans also play a fundamental role in climate regulation through a number of short-term and longer-term events such as the Pacific Decadal Oscillation (PDO) which provides a mechanism for moving heat from the tropical ocean to higher latitudes and out of the ocean into the atmosphere (Trenberth et al. 2002). In addition, we know that there is a dependency between climatic events in Greenland and Antarctica as a result of the dynamics of the deep Atlantic Meridional Overturning Circulation (AMOC): a strong AMOC is thought to deliver heat to the North Atlantic at the expense of the Southern Ocean. Therefore, a fluctuating AMOC strength results in climatic oscillations between the southern and northern hemispheres (Stocker and Johnsen 2003).
The effects of climate change on the Atlantic currents are unclear, though an investigation by Zickfield et al. (2006) into the response of the AMOC to climate change where 12 leading climate change scientists were interviewed, revealed that ‘all experts anticipate a weakening of the AMOC under scenarios of increase of greenhouse gas concentrations’. Further, as many as 8 of the experts predicted that the probability of triggering a total collapse of the AMOC through anthropogenic activity, was significantly different from zero (three predicted a probability >40%).
Consequences of a reduced AMOC are likely to include strong changes in temperature, precipitation distribution and sea level in the North Atlantic area (Zickfield et al. 2006).
The oceans are the life-source of Earth and these findings show just how apparent the human footprint is on them (and these are just a few examples!). It is abundantly clear that we need to do more to mitigate the impact of human activities on marine ecosystems if we are to slow down the inevitable changes that are occurring to our planet. Despite new studies, such as the one offered by Halpern et al. the stark reality is that we are, for the time being, simply unable to understand the full extent of humanity’s impact upon the oceans…only time will tell.
