Rising levels of greenhouse gasses increase the amount of solar energy absorbed by our atmosphere. This in turn warms both the atmosphere and the surface ocean, raising global temperatures, melting polar icecaps and altering the global hydrological cycle. From a geologic perspective, the concentration of CO2 (i.e. greenhouse gasses) in the atmosphere is naturally variable. Variation in the past has been due to plate tectonics and seafloor rifting, the drawdown of carbon from photosynthesis, and movement of deepwater in the ocean. The rate of atmospheric insolation, and by association global ocean temperatures, is directly tied to the carbon cycle of the planet.
In the modern world, atmospheric CO2 concentrations are now determined by human activity. Carbon-loading into the atmosphere immediately begins to unnaturally warm the surface of the planet. Climate models are used to generate predictions of how sea surface temperature (SST) will change through the 21st century; these models are built from telemetered satellite data, oceanographic instruments, and coupled ocean-atmosphere models. From such models, predictive SST maps are built to understand the spatial patterning of future warming.
Why does sea surface warming threaten marine ecosystems?
Just like terrestrial organisms that are cold-adapted or warm-adapted, organisms in the ocean are evolutionarily perscribed to specific temperature ranges. As global SST climbs out of the modern envelope, the warming of surface waters has the capacity to shift or decrease biogeographic range limits. This is because when organisms experience temperatures outside of the range they are adapted to, the result is often fatal. Marine invertebrate species range widely in their thermal tolerance limits and in their abilities to adjust in temperature-adaptive manners. Warm-adapted tropical species may live closer to their thermal tolerance limits than cold-adapted species, which means that tropical species may be much less capable of adapting to, and expanding their range because of, increasing seawater temperatures.
Coral reefs are an interesting case of how warming alters fundamental physiological processes in certain organisms. High temperatures disrupt the physiological symbiosis between corals and their algal symbionts. When experiencing high temperatures, the host coral expels the algal symbiont, and with it also expels most of the pigment within their tissue. Thus, the coral skeleton is white and the phenomenon is known as coral bleaching. Coral bleaching is very grave for reef systems. Bleached corals have lost there metabolic engines and very vulnerable to disease, damage and likely death. Entire reefs, when experiencing high temperatures, can bleach simultaneously; damaging or destroying a vast marine ecosystem that is centuries old.