As stated before, ice melting will lead to sealevel rises, both regionally and worldwide. A good overview of this subject is found at Wikpedia's review of sealevel change.
As an illustrative example of how a specific region would be affected, this predictive map of southern Florida shows the extent of inundation if sealevel were to rise just 5 meters (about 16 ft):
But is sealevel actually rising? Data from the last decade, using satellite imagery, points to a strong affirmation of this change:
Some other lines of evidence are shown in these figures, taken from the aforementioned Wipedia website:
The first shows sealevel rise trends for the last 125 years as measured at 23 marine gauges:
But this question can be raised: Have comparable rises been observed in the past?
From observations made at various islands and continental seaports, sealevel has steadily risen since the end of the last major continental glacial advance over the northern hemisphere; note that sealevel rise has approximately leveled off in the last few thousand years.
The trends in temperature and sealevel fluctuations over the last 900000 years is shown in this next diagram, which has an enlargement showing the changes during the last 140000 years. The impression is that there has been a cyclic pattern that makes it difficult to say that the current observations are unusual:
Over the last 500 million years or so, sealevel has undergone some broad patterns of change (as determined in a study supported by Exxon and in a separate study by Halam et al. that has used worldwide data and attempts to compensate for shifts of continents driven by plate tectonics). Higher general levels have occurred at the beginning of the Paleozoic and the end of the Mesozoic, with a minimum during the Pangaea breakup at the end of the Paleozoic. Note that the rise/fall trend during the glacial activity in the Neogene (N) is much less in magnitude than during earlier maxima.
The main conclusion one reaches after looking at the sealevel change data for a long interval of geologic time (assuming the patterns shown are generally accurate) is that short-term changes are highly variable but over millions of years trends of rise or fall proceed more gradually. The current rise may indeed be due to man's activities but alternate explanations (natural fluctuations, etc.) remain competitive. But it is clear that global warming is now taking place, owing probably to several causes, and as a result coastal areas are likely to be inundated (requiring either relocation of cities or emplacement of elaborate dike systems).
There is another aspect of the relationship between CO2 and ocean water that was brought to the writer's attention by an article in the July 3, 2010 issue of the Economist magazine. Entitled "The Other Carbon Dioxide Problem", it deals with ocean acidification. The gist of the information summarized in the article is recast in the next several paragraphs. A common theme throughout is the influence of feedback mechanisms on marine ecology. The concept of "feedback" is an important one, worthy of a quick synopsis taken directly from this Wikepedia website. Several sentences from that source are extracted here: "Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event/phenomenon (or the continuation/development of the original phenomenon) in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or loop, then the event is said to "feed back" into itself. Feedback is a mechanism, process or signal that is looped back to control a system within itself. Such a loop is called a feedback loop. In systems containing an input and output, feeding back part of the output so as to increase the input is positive feedback; feeding back part of the output in such a way as to partially oppose the input is negative feedback." An example of positive feedback is the role of water vapor in global warming: increased temperature produce more evaporation, hence more water vapor which, as a major greenhouse gas, itself contributes to further temperature rise. However, the same evaporation process can induce negative feedback if the vapor condenses to form clouds which shield the Earth's surface from heating by solar radiation, thus lowering temperature. Both modes of feedback will be highlighted at the end of the following paragraphs that consider ocean acidification.
The starting point in examining ocean acidification is simply the scientific fact that as the amount of CO2 in the atmosphere increases, more of that gas will, in an equilibrium system, react with H2O to form the weak carbonic acid H2CO3. More of that acid makes the ocean water more acidic. Increased acidity will result in the calcium carbonate of coral reefs being more readily attacked and dissolved.
Acidity is a variable that measures the concentration (commonly, in units of moles) of H+ ions in a aqueous solution. It is usually expressed in terms of pH (p is not a quantitative unit itself, and has no specific name). We will not discuss at length the meaning of pH but will refer you to the Wikipedia entry that treats it in detail. For now, as a "quickie" overview, just consider these points:
Molecules of water are for the most part stable but a small fraction will dissociate (break apart) in H+ (positive ions of Hydrogen) and OH- (negative ions of the Hydroxyl radical). In the equilibrium state, pure water thus follows this pattern for the fraction dissociated: H2O <--> H+ + OH-. The concentration of each of these ions in pure water, expressed in terms of pX is defined as pH = - log [H+] and pOH = - log [OH-]. Pure water has a pH of 7, in which there are equal amounts of H+ and OH-. For a more acidic solution, in which H+ exceeds any -OH, take the case where H+ = 0.0001M = 10-4; the log of 10-4 = -4; therefore pH = - log [H+] = - log (10-4) = - (-4) = +4 = pH of 4. M is the concentration in moles given in units of molarity. (For any substance, the number of atoms or molecules in a mole is Avogadro's number (6.02 x 1023) of particles. Defined exactly, it is the amount of pure substance containing the same number of chemical units that there are in exactly 12 g of carbon-12. For each substance, a mole is its atomic weight, molecular weight, or formula weight in grams. The number of moles of a solute in a litre of solution is its molarity (M); the number of moles of solute in 1,000 g of solvent is its molality.)
The pH of a variety of substances ranges from just above 0 to 14. This diagram shows some common substances.
The pH of seawater is on average around 8.1 to 8.2 and ranges between 7.5 and 8.5. The range is controlled mainly by seawater chemistry but is influenced by other variables such as biotic content and atmospheric factors. This is a recent map generalizing oceanic pH variability; note that there is also variability with depth:
The oceans consist not only of water but of dissolved salts. By far the must abundant are sodium and chloride ions that can precipitate out as halite or common salt. The oceans contain on average 35 parts per thousand (the rest is water) of dissolved salts (or, 3.5%). This diagram summarizes this chemistry:
The chemistry of the carbonates of calcium and magnesium is of special significance, as these make up the shells of most marine organisms. Carbonates are sensitive to the amount of CO2 in the water as this controls both the CO- available to the organisms and the amount of carbonic acid that can form and in turn can dissolve shells, etc. Consult this Seawater System chemistry website for a review of this topic. This next set of chemical equations summarizes the chemistry of CO2 as it interacts with water. The important things to note are 1) an increase in CO2 leads to increasing carbonic acid, 2) part of the carbonic acid dissociates in H+ and the bicarbonate ion HCO3-1, of which part dissociates into CO3-1 and another H+. Thus does the increase in carbon dioxide result in adding new H+'s to the oceans.
Through experimentation, the lowering of pH (increasing acidity) with progressive rise of CO2 levels is expressed in this plot:
That pH is actually being lowered is demonstrated by this plot (from The Economist article) which shows measurements of CO2 changes at Mauna Loa in Hawaii and pH off a seaport on the island of Hawaii; the cyclical 'wiggles' are the effect of summer-winter variations.
Using these recent trends as a guide, and taking into account calculations of expected increases in CO2 emissions from global warming models, this pair of diagrams shows projected effects to the year 2100:
A variant of this (from Turley et al, 2006) shows estimated average ocean pH's from 25 million years ago extrapolated into the future. If this projection is valid, the oceans will become more acidic than any time in that past.
What are the consequences of this change in CO2 and therefore ocean acidity? First off, corals in reefs are likely to be affected. This is a typical scene showing part of a coral reef:
The polymorph (same composition; different crystal structure) of Calcite (calcium carbonate) is Aragonite. That mineral is found in many marine organisms including corals and free-floating calciferous phytoplankton. This quadruplet of panels shows calculated and projected aragonite levels in the world's oceans for the time span between 1650 and 2100.
This information can be recast in a different way. The six panels below show Aragonite levels for different carbon dioxide amounts in the atmosphere, covering both past, present (350 ppm), and future values.
Lets now discuss the role of acidification in terms of feedback mechanisms. A rise in CO2 leads to a rise in acidity. If this is, in itself, a good thing, then this outcome is positive. Some organisms do flourish more when the seawater is more acidic than usual. Other organisms, however, are adversely affected if they make their protective shells out of carbonate of Ca and/or Mg; this is a negative feedback condition if the shells are more likely to dissolve if the acidity increase is too much. The mechanism: increased acidity increases H+ ions, and more bicarbonate ions, at the expense of decreasing carbonate ions (which can impede shell production in corals, oysters, foraminifera, etc.)
Add to the picture this situation: On land particularly, increased photosynthesis leads to more CO2 being extracted from the atmosphere, which should impede ocean acidification. However, many land animals release CO2 through respiration, partially restoring the balance; land plants also release CO2 when they decay. In the oceans some organisms at depth "respire" as a metabolic process making deeper ocean waters more acidic; these do not depend on photosynthesis for energy or bodily production. But most shallow water plant organisms (algae, diatoms, etc.) use penetrating sunlight to photosynthesize the CO2 into organic molecules (starting from sugars). These organisms are the "primary producers" in the oceans and are the central base that establishes and controls the food chain.
The main point alluded to in the above paragraphs is just that changes in CO 2 introduce a variety of positive and negative feedback mechanisms. Some consequences involve loss of hard parts in organisms - this is negative. Other consequences can lead to certain organisms increasing in numbers - this is positive. But the bottom line is this: While certain plants and animals may thrive if atmospheric carbon dioxide levels continue to increase, corals and other ocean dwellers are more at risk for maintenance or even survival if/when the CO2 levels lead to dangerous pH levels.
Returning to the theme of consequences of global warming on land surfaces: Another possible sign of global warming would be shifts in the distribution of both natural vegetation and food crops. Consider this next diagram which is a map pair showing, on the left, the current range of the common sugar maple with the shift that could occur from just the predicted temperature rise associated with a CO2 increase to 700 ppm, and, on the right, a more drastic withdrawal northward if soil moisture reduction is included. This happens simply because the climate zone that favors this maple is sensitive to specific temperature and moisture ranges.
This shift in vegetation is still being demonstrated. Measurements of green leaf distribution and other measures of vegetation (various Veg. Indices) over the last 20 years using satellite data have now revealed a substantial increase in the "greening effect". A study reported in the journal Science includes a map that shows the regions of the world that have experienced greater vegetation development and areas losing vegetation, largely as a result of climate changes:
In parts of the Northern Hemisphere, the greatest increase (red is highest) in vegetative cover seen in this summary diagram shows a distinct concentration in the northern parts of Europe, Asia, and North America. To the south, the first Spring leafing is now about 1 week earlier and Fall loss of leaves is almost a week later. The most likely explanation is the warming effect of greenhouse gases and the greater availability of CO2.
Even more affecting would be the changing conditions suited to supporting certain staple crops. A CO2 rise to 550 pm would redistribute crop yields worldwide for common grains, as shown below. Note that warming in the higher latitudes of the Northern Hemisphere would favor increases in crop production in Canada/Alaska, Scandinavia, and most of Europe and Russia but changes in South America and most of Africa would move towards drops in yield.
Other gases, including those that contain sulphur compounds, contaminate the atmosphere. One of the products of vegetation decomposition, fossil fuel burning, and even animal flatulation is methane (CH4). Although present only as traces near the surface and in the stratosphere, this gas rarely is concentrated locally at a toxic level. Using various sources of data, a NASA program has now produced a global map of surface and upper atmosphere distribution of methane:
UARS has measured summer and winter concentrations of several gas contaminants in both earth hemispheres. The first pair of plots on the left are temperature distributions. Note particularly the changes around the polar regions.
Other human activities may increase the rate of global change. One activity now grabbing attention is deforestation, whereby humans slash and burn, or just clear-cut, huge tracts of trees to use the land for agriculture or the wood for building shelters. As developers denude these large regions, biodiversity decreases, and land-use, water run-off patterns, and local weather phenomena change. Satellite remote sensing has produced dramatic images of progressive deforestation, as witnessed in these two scenes taken five years apart over the State of Rondonia in the Brazilian Amazon Basin by NOAA's AVHRR.
In recent years, awareness of the effect on global warming of changing landuse has come to the fore. The argument centers around this: Vegetation serves as a sink for carbon, which in turn influences the amount of carbon dioxide that re-enters the atmosphere. Although a complicated mix of positive and negative feedbacks, this changing cycle of carbon redistribution is strongly tied to the vegetation land cover that is affected by populations determining how the land must be used to support and feed the inhabitants.
This is neatly illustrated by a historical reconstruction of the clearing of the eastern half of the United States over an extended time frame: from 1850 to 1920 - approximating the period of greatest expansion of that country's people as the railroads spread westward from the Eastern Seaboard. Consider these diagrams:
The principal change was removal of natural growth, disturbing the pristine land cover, and converting much of the land to farming. Although the replacement of woodland vegetation with crops kept some of the capacity for carbon storage, the balance that applied to carbon dioxide release was modified. Similar results have occurred both in the U.S. and worldwide as Earth's human population has notably increased over the last 100 years. Worldwide vegetation cover is decreasing, reducing the capacity to intake carbon from the carbon gases being added to the atmosphere from burning of fossil fuels and other "smokestack" activities.