The entire earth’s climate is intimately tied to the behaviour of the oceans
Oceanographer Dr John Gould highlights the importance of the oceans in our climate, and the consequent need to observe them
Photo: NASA, GSFC
When Arthur C Clarke saw photographs of our planet taken by the Apollo astronauts he exclaimed that the earth had been misnamed; it should, he said, be called planet ocean. Indeed, from above the Pacific all one can see from space is white clouds and blue ocean, revealing the two fluids that govern our weather and climate. The case for Clarke’s pronouncement is even stronger when you realise that the oceans hold 97% of earth’s water, and land accounts for only 29% of its surface.
It is because of the oceans that Britain has mild winters and temperate summers, over-simplistically attributed to the Gulf Stream. Indeed the entire earth’s climate – past, present and future – is intimately tied to the behaviour of the oceans.
Exploring the oceans
Britain’s close association with the oceans has been the stimulus for exploration aboard ships with such evocative names as Endeavour, Challenger and Discovery – names also chosen, appropriately, for the US space shuttle fleet. The global voyage of HMS Challenger in the 1870s started an era of scientific exploration, first to describe the oceans, then to learn how they ‘work’, and most recently to focus on their role in climate.
Ocean scientists face huge obstacles in gathering their information. Anyone can see features on the moon a quarter of a million miles away, but only a select few have glimpsed the deep sea floor. The oceans are deep and remote, and sea water is corrosive and largely impenetrable to light and radio waves. How those obstacles were overcome is the focus of a recent book, Of Seas and Ships and Scientists, describing the establishment, following World War II, of the UK’s National Institute of Oceanography. Thankfully, since the 1980s new observational tools have been developed that are starting to match the enormity of the task.
How are the oceans important for our climate?
Sea water’s physical properties govern the role played by the oceans in influencing our climate. Evaporation from their surface produces the moist atmosphere that gives the rain that sustains life, while sea water’s high heat capacity – a slab 3 metres thick holds as much heat as the entire atmosphere above it – makes the oceans very effective at storing heat and transporting it around the planet. Sea water’s density depends on both temperature and salinity, and the interplay of these factors means that water moves both horizontally and vertically in the global thermohaline circulation. This circulation allows the oceans to absorb properties from the atmosphere (notably heat and carbon dioxide) and lock them away in the depths for hundreds of years, so buffering earth from rapid climate fluctuations.
To understand how our climate might change we need to collect ocean observations to match those we have from the atmosphere, and to feed them into state-of-the-art computer models. Research ships are still a mainstay, but there are few of them and they cannot make all the measurements needed.
A new era of ocean observing started in 1978 with the launch of the first satellite devoted to the oceans. SeaSat’s radar sensors could see through clouds and allowed round-the-clock monitoring. Since then similar satellites have measured the ups and downs of the oceans’ surface, not just the changes due to waves and tides but also changes in sea level across currents such as the Gulf Stream and the subtle effects of warming and cooling – and these changes, amazingly, are detected to an accuracy of just a few millimetres.
Satellites, though powerful, can only measure the external ‘symptoms’ of what is going on inside the oceans. To look below the surface, robotic probes now complement ship- based observations and routinely measure temperature and salinity. Most are in the 3,000-strong fleet of Argo profilers, each one drifting with the deep currents and regularly rising and sinking through the oceans’ top 2 kilometres and beaming their data to climate- monitoring centres. Others are underwater gliders capable of navigating themselves and probing the important but difficult-to-access regions between the deep ocean and the continental shelves and around and under ice-covered areas.
Near the equator, where interactions between the warm ocean and the atmosphere are most energetic, a network of 120 fixed buoys monitors the upper ocean and weather conditions to detect changes that, at their most extreme, manifest themselves as El Niño, a disruption to normal climate patterns that affects weather not just near the equator but around the world.
Designing and launching satellite missions and maintaining global in-water observing systems is beyond the capability of any single nation. International collaboration, as exemplified by the Argo fleet of profilers maintained by thirty nations (at a total annual cost of £26 million), is the way ahead. Argo data are freely and openly shared.
Ocean observations – why do they matter?
Satellite measurements confirm the accelerating rate of sea-level rise in tidal records extending back to the nineteenth century but, most importantly, can now show where sea-level rise (and fall) is greatest. The average rise of 30 centimetres since the pre-industrial era is largely due to the oceans’ warming, and to melting and freezing of icecaps and glaciers. Worldwide, 150 million people live less than 1 metre above the present high tide level. For them, and for people in Britain with homes that are vulnerable to coastal flooding and erosion, rising sea level is a matter of great concern. Steps to reduce coastal flooding and erosion cost £745 million for England and Wales in the year to March 2011.
The same satellites have measured changes in the strength and position of ocean currents, which transport vast quantities of heat around the globe and redistribute it away from the equator towards the poles. Since 2004 a joint UK/USA network of sensors has revealed unexpected large variability in this transport. Major changes in ocean heat transport have been implicated in the onset of ice ages and interglacial periods such as the one in which we now live.
Just as the atmosphere has warmed since the Industrial Revolution, so have the oceans, most rapidly near the surface (around 0.1 °C per decade). Over 90% of the heat associated with earth’s recent warming is stored in the oceans, and they have also absorbed a quarter of the carbon dioxide released by our modern lifestyle. These two factors have greatly limited the impact of human-induced global warming. But there is a down-side, for increasing temperatures and acidity (dissolved carbon dioxide is acidic) threaten living organisms, particularly coral reefs.
The moisture that falls as rain has evaporated from the oceans, and the salinity at the ocean surface reflects the exchange of water between the atmosphere and the ocean and can be used as a type of rain gauge. While the patterns of salinity are unchanged over the past forty years the fresh areas have freshened and the salty areas have become more saline. This suggests that evaporation and precipitation have increased, implying a strengthening of the global water cycle consistent with an increase in both floods and droughts. We also now have a better understanding of the mechanisms that cause El Niño events, and using advanced computer models we can now predict their onset six months in advance. These predictions are used by farmers, planners, the emergency services, commodity traders and the insurance industry.
In forty years we have moved from an era of curiosity-driven research to one in which the understanding we have gained and the observations we have made are serving society in an enormous number of areas – fisheries, oil and gas exploration, search and rescue, coastal protection, oil-spill tracking, merchant shipping and naval operations, as well as climate applications. Ocean forecasts are now being made routinely.
The BBC’s iconic Blue Planet series showed fascinating images of life below the ocean surface. But how can we convey to non- specialists the wider importance of the oceans? Fortunately, in parallel with the revolution in ocean observations has come a revolution in communication: the internet is now an integral part of our lives.
Capitalising on these two strands, space agencies post stunning images of the oceans on their websites, and open access to data from projects such as the tropical buoy array provides an insight into the ocean world (www.pmel.noaa.gov/tao/elnino). Moreover, free access to Argo data means that anyone can now ‘see’ the conditions deep inside the remotest parts of the ocean and be guided on a tour of the oceans’ interior (www.noc.soton.ac.uk/o4s/euroargo).
The advances in understanding that have come from new observations herald an era in which the oceans will play a bigger role in all our lives. Climate is just one aspect, with useful seasonal forecasts a key objective – ‘ Will the coming winter be wet/dry, cold/ warm, windy/calm?’ In addition to providing food, the oceans are also becoming a source of energy, minerals and pharmaceuticals, and changes such as an ice-free Arctic would have profound impacts.
Despite the progress, more remains to be done and to be discovered, and all of this will require continued commitment by governments and further international collaboration. Making the public aware of the oceans’ role in their lives is a key step towards that goal.