Environment Counts | The oceans and climate change – An EC perspective on the evidence presented in IPCC’s 5th Assessment Report
Author: Rick Higgins – Published At: 2014-11-16 11:02 – (3110 Reads)
The Fifth Assessment Report (AR5) of the IPCC states there is strong evidence that four global measures of ocean change have increased since the 1950s: the inventory of anthropogenic carbon dioxide, global mean sea level, upper ocean heat content, and the salinity contrast between regions of high and low sea surface salinity.
About 93% of the excess heat energy stored by the earth over the last 50 years is found in the ocean and changes in ocean heat content dominate changes in the global energy inventory. The report states that global mean sea level (GMSL) rose by 0.19 (0.17 to 0.21) m over the period 1901 to 2010. The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). The report concludes with high confidence that the observed patterns of change in the subsurface ocean are consistent with changes in the surface ocean in response to climate change and natural variability and with known physical and biogeochemical processes in the ocean. IPCC AR5 Chapter 3: Observations: Ocean
This article is one of a series of eight providing an EnvironmentCounts.org (EC) perspective on various aspects of IPCC’s AR5. Each article focuses on the primary data and related evidence presented and specifically excludes coverage of projections, as per EC’s editorial policy and guidelines.
Importance of the oceans in climateÂ change
The oceans influence global climate by storing and transporting large amounts of heat, freshwater and carbon, and exchanging these properties with the atmosphere. The ability of the oceans to store vast amounts of heat reflects the large mass and heat capacity of seawater relative to air and the fact that ocean circulation connects the surface and the interior ocean.
- About 93% of the excess heat energy stored by the earth over the last 50 years is found in the ocean.
- More than three quarters of the total exchange of water between the atmosphere and the earthâ€™s surface through evaporation and precipitation takes place over the oceans.
- The ocean contains 50 times more carbon than the atmosphere.
- At present the oceans are acting to slow the rate of climate change by absorbing one quarter of human emissions of carbon dioxide from fossil fuel burning, cement production, deforestation and other land use change.
- Changes in the oceans may result in climate feedbacks that either increase or reduce the rate of climate change.
Climate variability and change on time-scales from seasons to millennia is therefore closely linked to the ocean and its interactions with the atmosphere and the cryosphere. The large inertia of the oceans means that they naturally integrate over short term variability and often provide a clearer signal of longer-term change than other components of the climate system.
Ocean warming and the Earth’s energyÂ inventory
Ocean warming dominates the global energy change inventory. Melting ice (including Arctic sea ice, ice sheets, and glaciers) and warming of the continents and atmosphere account for the remainder of the change in energy. The following chart from chapter 3 of AR5 plots energy accumulation within components of the Earth`s climate.
|Global energy accumulation in Earth`s climate system|
|The above chart plots energy accumulation in zetta-joules (ZJ) (See note below on zetta-joules).|
|Ocean warming (heat content change) dominates, with the upper ocean above 700 m (light blue), contributing more than the deep ocean below 700 m (dark blue).|
|Ice melt (light grey); for glaciers and ice caps, Greenland and Antarctic ice sheet estimates starting from 1992, and Arctic sea ice estimate from 1979â€“2008.|
|Continental (land) warming (orange).|
|Atmospheric warming (purple); estimate starting from 1979.|
|Uncertainty in the ocean estimate also dominates the total uncertainty (dot-dashed lines about the error from all five components at 90% confidence intervals).|
A zetta-joule (ZJ) = 10**21 joules (or 1 sextillion joules) or 300 billion megawatt hours. The annual global energy consumption by all humans for all uses in 2008 was approximately 0.5 ZJ or 150 billion megawatt hours.
The report concludes:
- Warming of the ocean accounts for about 93% of the increase in Earth’s energy inventory between 1971 and 2010.
- Warming of the upper (0â€“700 m) of ocean accounts for about 64% of the total increase in Earth’s energy.
- It is likely that warming of the ocean between 700 and 2000 m contributed about 30% of the total increase in global ocean heat content between 1957 and 2009.
- It is virtually certain that the upper ocean (0-700 m) warmed from 1971 to 2010.
- In the upper 75m near the sea surface warming has been strongest at >0.1Â°C per decade between 1971â€“2010.
- At the depth of 700m warming has been lower, decreasing to about an increase of 0.015Â°C per decade.
- The estimated net increase in Earth’s energy storage between 1971 and 2010 is 274 (196 to 351) zetta-joules (ZJ).
Salinity and freshwater contentÂ change
Ocean salinity change is important because salinity changes, like temperature changes, affect circulation and stratification, and therefore the oceanâ€™s capacity to store heat and carbon as well as to change biological productivity. Because the total amount of salt in the ocean does not change, the salinity of seawater can only be changed by addition or removal of fresh water. About 85% of the evaporation and 77% of global precipitation occurs over the ocean. The horizontal salinity distribution of the upper ocean largely reflects this exchange of freshwater, with high surface salinity generally found in regions where evaporation exceeds precipitation, and low salinity found in regions of excess precipitation and runoff.
The salinity of near-surface waters is changing on global and basin scales, with increase in the more evaporative regions and decrease in the precipitation-dominant regions in almost all ocean basins. The spatial pattern of surface salinity change is similar to the distribution of surface salinity itself: salinity tends to increase in regions of high mean salinity, where evaporation exceeds precipitation, and tends to decrease in regions of low mean salinity, where precipitation dominates.
The following table summarizes some of the conclusions of AR5 regarding ocean salinity of relevance to climate.
|It is very likely that regional trends have enhanced the mean geographical contrasts in sea surface salinity since the 1950s.|
|Saline surface waters in the evaporation-dominated mid-latitudes have become more saline.|
|The relatively fresh surface waters have become fresher in rainfall-dominated tropical areas and from increased snow and ice melt in the polar regions.|
|It is very likely that the interbasin contrast in freshwater content has increased: the Atlantic has become saltier and the Pacific and Southern oceans have freshened.|
|It is very likely that large-scale trends in salinity have occurred in the ocean interior.|
|It is likely that both the subduction of surface water anomalies formed by changes in evaporation-precipitation and the movement of density surfaces due to warming have contributed to the observed changes in subsurface salinity.|
Sea levelÂ change
The report states with high confidence the rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia. Over the period 1901 to 2010, global mean sea level (GMSL) rose by 0.19 (0.17 to 0.21) m. There is very high confidence that maximum global mean sea level during the last interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at least 5 m higher than present, and high confidence that it did not exceed 10 m above present. During the last interglacial period, the Greenland ice sheet very likely contributed between 1.4 and 4.3 m to the higher global mean sea level.
Sea level change conclusions in AR5 report
- Over the period 1901 to 2010, global mean sea level rose by
|1901 to 2010||0.19||(0.17 to 0.21)||meters|
- It is likely that the rate of GMSL rise has continued to increase since the early 20th century.
- It is very likely that the mean rate of GMSL rise was
|Between 1901 and 2010||1.7||(1.5 to 1.9)||mm/yr|
|Between 1971 and 2010||2.0||(1.7 to 2.3)||mm/yr|
|Between 1993 and 2010||3.2||(2.8 to 3.6)||mm/yr|
- There is high confidence that over the period 1993 to 2010 GMSL rise is consistent with the sum of the observed contributions from
|Ocean thermal expansion||1.1||(0.8 to 1.4)||mm/yr|
|Glaciers||0.76||(0.39 to 1.13)||mm/yr|
|Greenland ice sheet||0.33||(0.25 to 0.41)||mm/yr|
|Antarctic ice sheet||0.27||(0.16 to 0.38)||mm/yr|
|Land water storage||0.38||(0.26 to 0.49)||mm/yr|
|Total||2.8||(2.3 to 3.4)||mm/yr|
The following chart from chapter 3 shows trends in GMSL rise from 1900 to 2000 from four different sources with the shading representing the 90% confidence area.
Changes in oceanÂ biogeochemistry
The oceans can store large amounts of carbon dioxide. The reservoir of inorganic carbon in the ocean is roughly 50 times that of the atmosphere. Small changes in the ocean reservoir can have an impact on the atmospheric concentration of CO2. The ocean also provides an important sink for carbon dioxide released by human activities, the anthropogenic CO2. Currently, an amount of CO2 equivalent to approximately 25% of the total human emissions of CO2 to the atmosphere is accumulating in the ocean.
|It is very likely that the global ocean inventory of anthropogenic carbon increased from 1994 to 2010.This is based on high agreement between independent estimates using different methods and data sets (e.g., oceanic carbon, oxygen, and transient tracer data).|
|It is very likely that oceanic uptake of anthropogenic CO2 results in gradual acidification of the ocean. The pH of seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. The observed pH trends range between â€“0.0014 and â€“0.0024 per year.|
|High agreement among analyses provides medium confidence that oxygen concentrations have decreased in the open ocean thermocline* in many ocean regions since the 1960s.|
|The general decline is consistent with the expectation that warming-induced stratification leads to a decrease in the supply of oxygen to the thermocline* from near surface waters, that warmer waters can hold less oxygen, and that changes in wind-driven circulation affect oxygen concentrations.|
|It is likely that the tropical oxygen minimum zones have expanded in recent decades.|
*The thermocline is the transition layer between the warmer mixed layer at the surface and the cooler deep water layer.
The following graph indicates the increase in the global inventory of anthropogenic CO2 from a base 1950 to 2010. (1PgC = 1 billion metric tonnes of Carbon)