Decades into the industrial revolution, the HMS Challenger Expedition sailed into the sea, looking for answers to questions that still intrigue oceanographers to this day. Little did these pioneers of ocean sciences know that their measurements will be used 140 years later by Scripps Oceanography Researcher Dean Roemmich to measure the human-induced warming of the world's oceans since the mid-1800's.
One of the most under-appreciated facts in climate change is the fate of the energy trapped by greenhouse gas emissions from human activities. Human activities are releasing nearly 10 Gegatons of Carbon (about 36 Billion tons of CO2) into the atmosphere every year, driving atmospheric CO2 concentrations to 400 parts per million (ppm) from their original preindustrial levels of 280 ppm. This increase in CO2 and other greenhouse gases concentrations traps additional energy in the earth's climate system. What happens to this "extra" energy (0.5-1 watt/m2) remains a mystery to many outside the field of climate and ocean sciences.
Since 1955, over 90% of the excess heat trapped by greenhouse gases has been stored in the oceans (Figure from IPCC 5th Assessment Report). The remainder of this energy goes into melting sea ice, ice caps, and glaciers, and warming the continents's land mass. Only the smallest fraction of this thermal energy goes into warming the atmosphere. Humans thus, living at the interface of the land, ocean and atmosphere, only feel a sliver of the true warming cost of fossil fuel emissions.
This 90% of extra heat taken up by the ocean is mostly in the upper 700 meters (m) layer (about 60% of total excess heat), while 30% is stored in layers deeper than 700 m (IPCC 5th Assessment Report). The ocean absorbs most of this "anthropogenic heat" because:
- Water has a high heat capacity: It takes much more heat to warm 1 liter of water than it does to warm the same volume of air (or most other substances).
- The ocean is deep: The world's oceans cover 71% of the earth surface and are about 4 km deep on average. This represents a tremendous reservoir of heat.
- The ocean is dynamic: Heat, carbon, oxygen and various other quantities exchanged with the atmosphere are mixed throughout the ocean through currents, internal waves, eddies, and various other circulation mechanisms.
The largest changes in ocean temperatures were observed in the upper 75 m, due to closer proximity to the atmosphere and the large mixing within this layer (IPCC 5th Assessment Report). As we trap more energy in the earth climate system, heat penetrates further into the ocean. Two important geographic areas where the atmosphere "communicates" with deeper layers of the ocean are the North Atlantic and the Southern Ocean. Because of their distinct atmospheric conditions and geographic settings, surface waters near the poles can be buried into deeper layers, bringing along their heat signatures, thus warming the interior of the ocean.
The ocean has warmed. Where, how much, how quickly did it warm, and how much heat will it absorb in the future, are questions that send satellites orbiting around the globe and take oceanographers to the far reaches of the sea. Measuring the magnitude and rate of ocean heat uptake is a very complex and challenging task that requires enormous observational and modeling efforts (e.g. ARGO program). These new measurements provide an unprecedented peek into the ocean's inner behavior. Measuring heat content changes below 2000 m however has been sparse due to the critical technical challenges encountered in deeper layers (e.g. high pressure, longer profiling range), and measuring the very small temperature changes required to contribute significantly to changes in heat content of these large volumes of water.
In one of his last publications before his death in 1957, Carl-Gustav Rossby, a legendary atmospheric scientist, elegantly summarized the importance of the ocean within the climate system as: "Anomalies in heat probably can be stored and temporarily isolated in the sea and after periods of the order of a few decades to a few centuries again influence the heat and water-vapour exchange with the atmosphere."
Implications for Policy
As the scientific and policy community shifts its attention to the climate's response to increased greenhouse gas emissions (a.k.a climate sensitivity), we must not underestimate the magnitude, variability, and uncertainty in the ocean's ability to store and exchange heat with the atmosphere, which in turn influences climate on a global scale. One such example is the naturally occurring heat exchanges during El Nino Southern Oscillations events [explained best by Trenberth and Fasullo 2010]. Another example is the highly discussed role of the deep ocean and natural variability in the recent warming hiatus period.
The complex interactions between continued emissions of greenhouse gases, consequent energy imbalance, and changes in the storage and transport properties of heat in the ocean will largely determine the speed and magnitude of longterm anthropogenic climate change impacts. These interactions have significant policy and economic implications, and must not be ignored in the climate policy discussions forum. As the climate negotiators are now shifting their focus towards reaching an agreement on appropriate stabilization targets and designing mitigation and adaptations strategies required to meet those targets, understanding and incorporating the highly important role of ocean as the most powerful climate change mitigator becomes of utmost importance.