Carbonic acid is a weak acid that is produced when carbon dioxide is dissolved in water.
As you probably know, our atmosphere has a lot of carbon dioxide in it. It is also thoroughly saturated with water. From this, we might deduce that we live in a rather acidic environment — and we do. Carbon dioxide is constantly dissolving into the water that surrounds us, forming natural carbonic acid. Rainwater is naturally a little acidic, and the oceans and other bodies of water are always absorbing carbon dioxide and turning it into carbonic acid. On a day-to-day basis, this doesn't really make any difference. Carbonic acid is very weak, and generally hurts no one. But...
Carbonic acid melts the Earth
You bet it does. Calcite (calcium carbonate) is soluble in acidic water (it actually undergoes a transformation into calcium bicarbonate). Calcite is present in many rocks, most notably in limestone and marble. Over the course of centuries and millennium, water and carbonic acid slowly eat away rocks, a process too slow to see, but very important on a geological timescale. The carbonic acid breaks down not only strata at the surface, but does some serious damage underground, forming caves and karst topography.
But the acid isn't just a destructive force; it also creates cave formations such as stalactites, rock flows, soda straws and many others. Much of the water that soaks into the ground passes through biodegrading plant material, resulting in particularly high levels of carbonic acid. The higher levels of acid means that it can dissolve more calcite. When the water flows into an open cave the calcium bicarbonate will offgas carbon dioxide into the air, leaving calcite to precipitate out of the water. This calcite is left behind as the water drips, filling caves with flowing rock sculptures.
This may be the reason that the most interesting cave formations are found in warmer, wetter parts of the world, or in regions that were once warm and wet. These are the areas where there is the most composting plant matter, and thus the highest concentrations of carbonic acid in the groundwater. This is also why sinkholes and other karst features appear in these areas.
Carbonic acid flows in my blood
Yours too — carbonic acid is the primary acid buffer controlling the pH balance in the blood plasma of mammals. Many chemical reactions in your body depend on a very specific pH balance, but at the same time the level of cellular metabolism changes the pH balance.
When our cells exhale carbon dioxide into the bloodstream a large percentage of it is taken up into the red blood cells — but not carried directly to the lungs for disposal. Instead, it's combined with water to form carbonic acid, and then combined with more water, breaking the carbonic acid (H2CO3) into hydrogen carbonate (AKA Bicarbonate, HCO3-) and a positive ion (a proton, H+). The 'natural' rates of carbonic acid formation are much too slow to provide an effective buffer system, so this reaction is tremendously sped up by an enzyme called carbonic anhydrase that lives in your red blood cells.
The H+ produced is bound to the haemoglobin in the red blood cell, but the hydrogen carbonate slips out into the bloodstream (the red blood cell will absorb a Cl- ion from the plasma to keep its electrical balance from becoming too acid). Once the red blood cells get to the lungs they pick up oxygen and drop the H+. The H+ combines with the hydrogen carbonate to form carbonic acid, which pops out of the blood stream and into the alveoli of the lungs in the form of carbon dioxide.
This fancy dance keeps enough hydrogen carbonate (a base) circulating in the bloodstream to keep any spikes of acid muted. This is important — a healthy human body needs to have a pH balance between 7.35 and 7.45. When the blood becomes too acid, it is known as acidosis; too base and you have a condition called alkalosis. The body deals with these conditions by simply adjusting your rate of respiration, which changes the carbon dioxide level of your blood. On a much slower scale, the kidneys also adjust both hydrogen carbonate and H+ levels in the blood.
Carbonic acid poisons the oceans
Nothing tricky here; water plus carbon dioxide equals carbonic acid. Carbonic acid has been doing its part to keep down the pH balance of the oceans since the beginning of time (the oceans are naturally alkaline). In recent decades carbon dioxide emissions have been increasing; this is good news for spelunkers, but bad news for the environment. Not only is carbon dioxide a greenhouse gas, leading to global warming, but it's also mixing with the oceans at an accelerated rate, loading it up with excess carbonic acid. This process is known as ocean acidification.
If you recall, calcite dissolves in carbonic acid. As it happens, many types of oceanic plankton and other small animals use calcite and other soluble minerals to build their shells. coccoliths, planktic foraminifera, red algae, brachiopoda, echinoderms, bryozoa, corals of all types, and perhaps even bivalves (oysters and such) will be affected by carbonic acid attacking their internal and/or external structures. Other species, such as squid and whales may find it harder to keep their blood acid levels down. And a change in ocean chemistry might break down molecules, setting toxic metals free and destroying important nutrients. Even if the direct damage is limited to plankton, this still means massive disruption to the oceans' food chains.
Needless to say, this is bad, but we really have no idea how bad it might be. We don't know very much about any individual species reaction to increased acidity, and we know even less about the resiliency of oceanic ecosystems. We also don't know how other factors — such as a rise in oceanic temperatures — will interact with the acidification effects. Things could suddenly collapse in the next 20 years, leaving us without any sizable fish (or sea mammal) populations. Or things could all work out very nicely, Gaia having gone through any number of high-carbon, high-temperature shifts in the past few hundred million years. It will all be one splendid surprise for us humans.
Regardless of how much faith one wishes to place in the Earth goddess, the amount of carbon being taken into the oceans is really quite worrying. In the last two centuries the average pH of the oceans has dropped by 0.1. This doesn't sound like a lot, but it represents a thirty percent increase in hydrogen ions (H+). Scientists are predicting serious losses in species' habitats by 2030, and then onward into the foreseeable future. We are pumping more carbon dioxide than ever into our environment, and even if we were to stop today it would take decades for the effects of our actions on the oceans to become apparent.
References:
http://en.wikipedia.org/wiki/Carbonic_acid
http://www.nps.gov/cave/planyourvisit/upload/cave_geology.pdf
http://creationwiki.org/Stalactites_can_grow_very_rapidly
http://www.nps.gov/ozar/forteachers/speleothems.htm
http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/virtmap/index.html
http://scifun.chem.wisc.edu/CHEMWEEK/BioBuff/BioBuffers.html
http://en.wikipedia.org/wiki/Bicarbonate
http://en.wikipedia.org/wiki/Acidosis
http://en.wikipedia.org/wiki/Carbonic_anhydrase
Barron's How To Prepare For The MCAT, by Hugo R. Seibel, 2006
http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_02.htm
Intravenous Infusion Therapy for Nurses: Principles & Practice By Dianne L. Josephson, 2004
http://www.truthout.org/cgi-bin/artman/exec/view.cgi/61/20787
http://en.wikipedia.org/wiki/Ocean_acidification
http://www.publications.parliament.uk/pa/cm200607/cmselect/cmsctech/470/470we08.htm