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Appendix A. Climate Change in the Oceans

This appendix has been adopted by the CMOS council and will be submitted to CMOS members at the 2015 Annual General Assembly.

Ocean Temperature

Ocean temperature measurements have been made routinely at some stations since the 1950's. We have clear evidence that the ocean is warming and that this warming extends deep into the ocean interior. A given amount of heat changes the temperature of seawater less than that of air and thus, although the temperature changes in the atmosphere and ocean are comparable, the oceans have accumulated 45 times more heat than the atmosphere.

Human-induced warming is larger near the poles and so Canadian oceans have seen larger temperature increases than the global average. The Arctic Ocean is warming particularly fast as during the summer larger regions of the ocean are ice-free. Ice-free regions are less reflective to sunlight and absorb more of it.

Changes in ocean temperature influence the geographic range and seasonal timing of organisms in the ocean at all levels of the food chain from photosynthetic phytoplankton to fish and whales. Temperature is not the only determinant of habitat for ocean biota, so a simple shift northward of whole ecosystems is not expected. Species redistributions and seasonal timing changes will impact prey/predator/competitor interactions leading to substantial changes in Canadian waters.

Sea level rise

Sea level is currently rising at a rate of around 3 mm per year due to thermal expansion and melting glaciers and ice sheets. To a reasonable first approximation this is a globally uniform trend if considered on a long enough time scale (e.g., more than 30 years), except for glacial rebound effects. Because of adjustment of the land to melting of the great ice sheets of the last Ice Age, coastlines also rise and fall, in some regions mitigating sea level rise and in others accelerating it. Extrapolating the present global-average rate of sea level rise rate gives an increase of 30 cm (one foot) over 100 years. However, it is important to consider that even with this limited rise, what is considered an extreme storm surge at present will occur much more frequently (e.g., the 'once in 100 years' event might occur every 20 or 25 years and the 'once in 50 years' event every 5-10 years) even if storm frequency and intensity is unchanged.

We do not know how much freshwater will be lost from the Greenland and Antarctic ice sheets or how fast. It is likely that even if complete loss of the ice sheets occurs (resulting in more than 60 m of sea-level rise), it will take hundreds of years. For the next half century or so, the probability of a significant ice-sheet contribution to sea level rise is small, but it should be kept in mind that our 'baseline' estimates (thermal expansion plus wastage of smaller, low- and mid-latitude glaciers) are a lower limit.

Sea ice

Sea ice extent has been observed globally and continuously since the dawn of the modern “satellite era” of Earth observation in 1978-79. Summertime Arctic sea ice has shown a consistent downward trend throughout the data record, and the trend is accelerating. While natural variability is an important influence on short term variations, there is no known natural process that could produce the overall trend.

Climate models project a seasonally ice free Arctic sometime in this century, but with natural year to year variability it could occur as early as the 2020s. The record low ice extent observed in September 2012 (3.4 million square kilometres) was only about half of the 1979-2000 average for September. There is also a decline in ice thickness and an attenuation of the area of the oldest, thickest multiyear ice.

The disappearance of sea ice is already having impacts on Arctic residents and wildlife. It is also important to consider that in Canada sea ice occurs outside the Arctic, notably in the Gulf of St. Lawrence and the Labrador Sea. Loss of sea ice can exacerbate ocean acidification and impacts of sea level rise on coastal infrastructure. It will also open new areas of the Arctic to shipping and resource extraction, which will bring economic benefits and also new environmental risks.

Stratification

The ocean is stratified, with less dense water, warmer or fresher or both, above more dense water. As the Earth warms, ocean stratification will increase, in general, due to surface warming and, in some areas, an increased surface input of freshwater. This increase in stratification has already been observed in a number of regions of the ocean.

In many regions of the ocean, phytoplankton growth is limited by lack of nutrients. Increased stratification will tend to decrease the transport of nutrients into the sunlit surface layers of the ocean and thus decrease phytoplankton growth.

Ocean acidification

About 40% of the fossil fuel CO2 emitted since the industrial revolution has been taken up by the oceans. CO2 makes seawater more acidic and corrosive of calcium carbonate (CaCO3) minerals that a wide variety of marine animals use to build their shells and exoskeletons. Ocean acidification is not a consequence of climate change, but a separate and parallel consequence of the atmospheric CO2 increase. The surface ocean does not naturally dissolve CaCO3, but the deep ocean does because it contains more CO2 in solution. The depth of this change is becoming shallower globally. This threshold is naturally shallow in mid-to-high latitudes and particularly in the North Pacific, making some Canadian waters especially vulnerable. In addition, freshwater inputs from ice melt reduce the capacity of seawater to buffer against pH changes caused by CO2, and near-surface waters able to dissolve CaCO3 have already been observed in the Canadian Arctic.

Ocean deoxygenation

Marine animals (except air-breathing birds and mammals) depend on oxygen dissolved in seawater, and the concentration of oxygen will decline in a warming climate. The only source of oxygen in the subsurface ocean is mixing and downward transport of surface water. The amount of oxygen that can dissolve in seawater is mainly a function of temperature, so as sea surface temperature increases, the "time zero" concentration (the concentration when the water mass leaves the surface) declines. Increased thermal stratification reduces the rate at which oxygen is transported to the subsurface ocean, but may also reduce the consumption of oxygen by organisms in the subsurface ocean. The downward flux of organic material is expected to decline slightly in a more stratified ocean. Presently, subsurface oxygen concentrations appear to be declining at a much greater rate than can be explained by temperature effects on solubility alone, but the data records are still too short to clearly separate long-term trends from natural variability.