We've now seen HOW climates form and WHY they change. Now it is time to look at what those changes mean for us. This week we look at the effects of climate change on weather systems and water availability; the next few weeks we look at the effects on the biosphere.
Attribution: Is It Really Anthropogenic?
But first, how do we know the new carbon dioxide is really anthropogenic? Is there any way to show that it is a byproduct of fossil fuels rather than modern biomass? And a way to show that it is caused by the burning of carbon biomass rather than just volcanically-derived CO2?
Ancient Sources: The Suess Effect: modern biomass contains a measurable fraction of
In 1955 Austrian chemist Hans Suess recognized that we could see changes in the amount of
Organic Material Rather Than Volcanics: The Keeling, Jr., Curve: Carbon dioxide can come from a number of sources. Carbon dioxide, for instance, can be emitted by the eruption of volcanoes from sources in the mantle. Ralph Keeling (son of the Charles Keeling of "Keeling Curve" fame) figured a method to demonstrate that the new carbon dioxide was the result of oxidization of carbon. If you burn carbon-rich sources, two atoms of O are used up for each carbon dioxide molecule; thus, the total amount of O2 goes down over time. Of course, since O2 is vastly more common than CO2, such changes would be hard to test directly. So Ralph Keeling instead measured the ratio of the amount of O2 to N2 (ratios are often easier to measure with mass spectrometry than total amounts) over time. He found that there is a pronounced downward trend, and an annual oscillation mirror-imaging the CO2 oscillation in the standard Keeling curve.
So it is indeed burning fossil fuels which are being added to the atmosphere. How does this affect us?
Temperature and Heat
An obvious effect of increased levels of greenhouse gases in an increase in global temperatures over time. Indeed, in recent years Australia's meteorological services have had to introduce new colors on their temperature maps to keep the old color scale still in use (that is, so that the same shades on old maps reflect the old temperature ranges.) Similarly, the world has seen the 10 hottest years of the instrumental record all in the 21st Century (which is only 15 years old...)
But does this actually reflect the global situation? Could this be the urban heat island effect (the idea that most weather stations are near cities, factories, villages, and other places of human inhabitation and industry (and thus places where we are generating heat locally))? This is a testable idea, and has been tested. By deleting the stations near human habitation and industry we can eliminate the influence of the urban heat islands. We see that there is a very slight input from the urban heat islands, but that the vast majority of temperature change is global.
Keep in mind that due to the factors discussed for the last month, warming is not represented as simply raising the temperature everywhere by the same amount. Instead, some places warm more (esp. the Arctic), some less, and some regions (eastern North America, for instance) might actually cool (at least in the short term.
Something worth noting is that most of the warming actually goes into the mixed layer of the ocean, and not the atmosphere anyway. For a long time climatologists thought that the deep ocean wasn't undergoing much warming, but new data shows that heat content of the deep ocean is rising, too. (Keep in mind, the more the deep ocean warms, the less CO2 it will sequester.)
The ocean-covering sea ice of the Arctic is greatly reducing over the recent past. Sea ice, floating directly on the ocean, is decreasing in the Arctic. This does NOT affect global sea level, as this is floating ice. But the increasing open seas (especially during summer and early fall) in the Arctic Ocean affects regional albedo (less reflectivity, more heat: a positive feedback) and precipitation patterns (more evaporation).
An odd side effect of the drop in sea ice: the "Northwest Passage", a direct blue-water route from western Europe to China through the Arctic (long the question of European sailors wanting to avoid long circumnavigations) is actually now present.
Sea Level Change and Continental Glaciers
Even though melting sea ice doesn't change sea level, sea level is indeed rising. Why?
The main reason is thermal expansion; as water heats, it expands. Although the amount is only a tiny bit by volume, the ocean has a HUGE volume. Predictions show a rise of between 18-59 cm (7-23") by 2100 based on thermal expansion alone.
More important, though, is the melting of continental glaciers. As these melt, they drain into the oceans, producing sea level rise. Greenland ice is undergoing melting on the margins at quite a rapid rate. The total volume is reducing quite a bit (keep in mind, some of this ice has persisted for 10s or 100s of thousands of years). One reason of the high rate of ice loss is that melting ice forms rivers and channels that undermining the ice and allowing it to melt off at the edges.
It is not just Greenland, though. Antarctic ice is also melting a great degree.
Sea level rise from glacial melt can quickly raise sea level; after all, the glacial-interglacial cycles saw sea level up to 30 m or more, almost all of it from the sequestration or liberation of water. Estimates show that the 21st Century could see 75-190 cm (29.5-75") sea level rise from continental glacial melt. If Greenland lost all of its ice, it would be 6 m (20') total rise; even more when Antarctica melts.
Keep in mind that much of the world's population lives within a few meters of sea level, so these rises will require the displacement of billions of people and trillions of dollars worth of infrastructure. It would also mean the loss of important croplands; the disruption of marine transportation and ports; allow for cyclonic storms to impact further inland; and the loss (at least until the reestablish at a higher level) of wetlands important for biodiversity, economy, and control of water quality.
Montane Glaciers and Water Availability
Although they contain less total volume than the great continental glaciers of Antarctica and Greenland, glaciers in the mountains are highly important. For instance, the glaciers of the Tibetan Plateau and the Himalaya provide the sources for the great river systems of South, Southeastern, and Eastern Asia; in other words, they are fresh water for about 40% of our species.
However, this is not the only reason that many people will face drought in coming decades. Shifting weather systems will result in many parts of the Earth becoming drier on average, including major crop belts.
Less secure than all the above changes is the effect of global warming on storm intensity. There is some limited evidence of increasing numbers of intense cyclonic storms in recent years and theoretical reason to think that the future oceans will see more intense storms. This remains a matter of great debate, however.
Ocean Acidification: Global Warming's Evil Twin
We have already seen the effect in Earth's past of ocean acidification (such as the Paleocene-Eocene Thermal Maximum); we are seeing its effect now. Even if we blocked insolation to reduce heating, but still added greenhouse gases to the atmosphere, ocean acidification would still disrupt marine ecosystems.
Just as today if you turn off the heat of a burner the top of the stove and the kettle or pot doesn't immediately reach room temperature, so to if we stopped increasing forcing to the atmosphere we would still see decades of increasing temperatures and other effects. This is called climate inertia. The fact that the vast oceans are the major store of heat also means that there will be LOOOONGG period of equilibration between atmospheres and oceans. Thus, whether we want it or not (and we DON'T want it), we will face global change throughout the rest of our lives.