Global Warming Myths

Because global warming is such a politically controversial issue, there are many myths about the subject.  Some of these myths are based upon a nugget of truth, and others are complete fabrications.  It's time to get them debunked. 

Myth #1:  'They' predicted an ice age in the 1970s

This myth is based upon stories in TIME and Newsweek in the 1970s regarding global cooling.  It was even quoted by Republican Sen. James Inhofe, chair of the Environment and Public Works Committee, on the floor of the Senate. This myth appeals to some people because these 'predictions' have turned out to be so wrong. If 'they' were wrong in the 1970s on predicting global cooling, then 'they' could easily be just as wrong on recent global warming.  Some people therefore conclude that the science of global warming remains unsettled.

A better conclusion, however, would be that one should not rely on the mass media for scientific information if better sources are available (i.e. scientific studies themselves).
 

Recent studies of the scientific literature at the time have concluded that the supposed "global cooling" consensus among scientists during the 1970s is indeed a myth.

"Thomas Peterson of the National Climatic Data Center surveyed dozens of peer-reviewed scientific articles from 1965 to 1979 and found that only seven supported global cooling, while 44 predicted warming. Peterson says 20 others were neutral in their assessments of climate trends."

Among those scientists who predicted continued global cooling, some based their conclusions on the assumption that human aerosol emissions would continue to increase rapidly.  Up until 1970, aerosol emissions (primarily sulfur dioxide) were increasing rapidly.  Aerosols block sunlight, causing global dimming and thus global cooling, and were primarily responsible for the very slight global cooling from 1940-1970.

Some scientists were concerned that if these emissions continued to accelerate, they would overwhelm the warming associated with carbon dioxide emissions and continue to cool the planet.  However, many countries began to pass Clean Air Acts in response to the various problems associated with these emissions (acid rain, for example), and worldwide sulfur dioxide emissions have actually decreased since 1980.  In short, the scenario which was predicted as the possible cause of global cooling did not come to pass.

The lesson to be learned from this myth is that it's important to get our scientific information directly from scientists, rather than filtered through the media.  Media bias is one of the most common causes of misinformation with regards to global warming even today.  Some articles are too sensationalist, and others provide misinformation stemming from a political agenda.  This is why the Intergovernmental Panel on Climate Change (IPCC) is such a critical organization, and why it won a Nobel Prize.

Myth #2:  There is no scientific consensus on man-made global warming

This myth often stems from a misunderstanding of the term 'consensus'.  In this context, 'consensus' does not mean 'unanimity'.  The term generally means 'majority', but in the context of man-made global warming, it means 'vast majority' of agreement among scientists.

A 2009 study  concluded that based on a poll of over 3,000 Earth scientists, 82% agreed that "human activity is a significant contributing factor in changing mean global temperatures," including 97.4% of the 79 who listed climate science as their area of expertise and who also have published more than 50% of their recent peer-reviewed papers on the subject of climate change.

A nice compilation of the various groups which are in agreement about man-made global warming is available at Logicalscience.com.  Every so often, certain parties will attempt to compile lists of scientists who disagree with some aspect of the theory in an attempt to disprove the consensus.  The two most oft-cited lists are the Oregon Petition and Senator James Inhofe's list of 400.

In both cases, the lists were not limited to people with expertise in climate science, but essentially allowed anybody with any college degree (in the case of the Oregon Petition) or any scientific degree (in the case of Inhofe's list) to sign.  In the example of Inhofe's list, it included botanists, chemists, mathematicians, economists, and political scientists, to name a few.

When considering how many scientists work in these fields, 400 is a fraction of a percent and certainly does not disprove a scientific consensus on the subject.  Particularly since many on the list have no expertise or experience in climate science research.  The Oregon Petition has been found to be fraudulent, and a new version was recently circulated.

In short, it is quite apparent that there is indeed a scientific consensus on man-made global warming.  A nice description of what the consensus is can be found at RealClimate.

Myth #3:  One volcanic eruption emits more CO2 than humans have in [insert length of time here]

Because volcanic eruptions look so massive, some people assume they emit vast amounts of carbon dioxide.  However, studies have shown that on average, annual human CO2 emissions are at least 100 times greater than volcanic CO2 emissions. 

As one example, the Mount Pinatubo eruption emitted 42 to 234 million tons of CO2.  Humans emit 30 billion tons of CO2 per year, or approximately 80 million tons of CO2 per day.  In other words, humans emit approximately as much CO2 every day as the entire Mount Pinatubo eruption.

This particular myth has no basis in reality.  It's simply completely wrong.

Myth #4:  Mars and/or all the planets in our solar system are warming, so the Sun is causing global warming

There is a nugget of truth to this myth.  Our observations of Mars appear to indicate that the planet is warming.  However, we don't have a lot of data regarding the climate of Mars, and while some regions of the planet are warming, we aren't certain that the planet is warming as a whole.  If Mars is experiencing global warming, scientists believe it's due to dust storms darkening its surface, causing the planet to absorb more sunlight (a.k.a. decreased albedo).

A sub-myth is that both ice caps on Mars are melting.  In reality, while the southern polar cap is vaporizing, the northern cap is expanding.

A few other planets in our solar system appear to be warming.  For example, Pluto appears to be warming at a rate much faster than Earth (the opposite of what you would expect from a planet further from the Sun if the warming were due to solar effects), most likely because of its orbital cycles (seasonal changes).  However, of the many planets and moons in our solar system, the vast majority appear to be experiencing no significant temperature change.

Moreover, we don't need to look at other planets to measure changes in the Sun - we have satellites which have directly measured solar output for the past 30 years.  The satellite data shows that solar irradiance has not changed on average over that period, so it cannot be causing global warming on Earth or any other planet.

Myth #5:  Because the Earth's climate has changed naturally in the past, the current warming is natural

In order to know if the current climate change is the same as past climate changes, one must understand what caused the climate to change in the past.  This is discussed in the global warming and climate change causes wiki, which shows that the current warming cannot be explained by the effects which caused past natural climate changes.

Myth #6:  Global warming is just a massive hoax

This myth ignores the vast amount of scientific evidence supporting man-made global warming.  However, even ignoring the evidence, this myth is simply not possible.  As discussed in Myth #1, scientists have been predicting global warming since the late 1960s.  Thus perpetrating such a hoax would require millions of scientists all colluding over 4 decades, falsifying data with not one single credible person revealing the supposed hoax.  Most importantly, credible skeptical scientists like Richard Lindzen might disagree with the theory or its projections, but they don't argue that it's all a fraud.

Furthermore, ignoring the massive amount of collusion that such an immense hoax would require, and the fact that scientific frauds on far, far smaller scales have been revealed almost immediately, consider the fame that a scientist would get for disproving this hoax.  If global warming were really due to the Sun, and all scientists have been saying it's not, and another scientist proved that this was the case - you're talking fame, Nobel Prizes, etc. etc. Any scientist would love to disprove man-made global warming if it were wrong.

In short, it's simply impossible for man-made global warming to be a hoax.  This particular conspiracy theory has no basis in reality.

Myth #7:  Natural sources emit so much more CO2 than humans that we can't be causing global warming

One often hears the skeptics say that mankind is only producing about 5% of greenhouse gas emissions. Why that is not much at all, is it? This number is oft-quoted but it is very disingenuous. Out of all of the CO2 emitted from both natural and anthropogenic (man made) sources, it is true that only about 5% of the total, per year, is from those human activities.

However, this argument ignores ninth grade Earth Science class. The natural carbon cycle is central to our understanding of this issue.

Carbon dioxide is produced naturally here on Earth, by such means as animal respiration (or digestion, in the case of methane) and the decay of plant and animal matter. This carbon is then naturally removed or “recycled” from the atmosphere, mostly through the process of photosynthesis, or plant respiration.

For more then 10,000 years, since the end of the last ice age, the carbon cycle has been relatively stable. The amount of carbon dioxide that entered the atmosphere was roughly equal to the amount of carbon dioxide that was removed and the overall atmospheric CO2 concentration remained constant, at about 280ppm.

Climate change is about change. Humans have upset the natural budget of the carbon cycle. We have accomplished this through certain activities such as energy production (burning fossil fuels release large quantities of CO2 into the atmosphere) and deforestation. Humans are only adding carbon to the atmosphere, which is just half of the equation; mankind is not removing any carbon dioxide from the atmosphere. In addition, deforestation reduces the amount of CO2 that nature is able to remove from the atmosphere.

The next time someone argues that humans are only responsible for 5% of the atmospheric CO2, please mention the carbon cycle. They probably just forgot. Forgetting could be dangerous if they are talking about issues that are important. Suppose that I was budgeting to save money for something real nice. Suppose that I also only added essay up my incomes, but “forgot” to subtract my expenditures. I would have a pretty misleading amount in my bank account by the end of the year, right?

The atmospheric CO2 concentration has increased by about 37% since pre-industrialization times, and is now over 380 ppm.

 A single tree consumes up to 1 ton of CO2 while it is alive. Just imagine how much only one person can do for the world if he plant at least one tree in their life

A sub-myth is that if human CO2 emissions are causing global warming, we should just stop breathing, since we emit CO2 when we exhale.  The CO2 we breathe is part of the natural carbon cycle.  When we exhale, we're not adding any new carbon to the system - we're just circulating carbon that was already there.

Breathe easy - you're not contributing to global warming by doing so!

Myth #8:  CO2 is such a small fraction of the atmosphere, there's no way it's causing global warming

It's true that carbon dioxide currently accounts for just 384 parts per million (ppm) of the gases in our atmosphere.  That's 0.0384%.  Seems tiny, right?

The problem with this myth is that the concentration of CO2 isn't enough information to tell you how much global warming it's causing.  For example, arsenic is lethal at 15-30 ppm.  In this case you need to know the toxicity in addition to the concentration to know if the amount of arsenic is dangerous.  In the case of a greenhouse gas, you need to know its global warming potential.

The bottom line is that while 384 ppm or 0.0384% might sound small, that doesn't mean it's incapable of having a large effect.  In fact, nearly 99% of the Earth's atmosphere is composed of nitrogen and oxygen, which are not even greenhouse gases.

Myth #9:  Water vapor is a more important greenhouse gas than CO2

The nugget of truth to this myth is that water vapor comprises about 1% of the atmosphere (about 26 times more than CO2), and it accounts for the largest percentage of the greenhouse effect on Earth (approximately 36-66%).

However, the critical point is that water vapor cannot cause global warming.  This is because the amount of water vapor in the atmosphere depends entirely on the temperature of the atmosphere.  A warmer atmosphere will hold more water vapor.  You can emit as much water vaopr as you want, but unless the planet is already warming, the atmosphere won't be able to hold any more water vapor and it will just fall back to the ground as precipitation.  So while water vapor can act as a feedback and amplify global warming, it cannot initiate global warming on its own.

Myth #10:  Global warming stopped in 1998/2001/2003/2005/etc.

While it's true that in most surface temperature records 1998 remains the hottest year on record (NASA concluded that 2005 was slightly warmer), that doesn't mean that global warming has stopped.  The reason 1998 remains the hottest year on record is that it was an anomalously strong El Niño cycle year.

However, every year from 2001-2007 was hotter than every year in the 1990s other than 1998 (and every previous year on record).  Thus even though there has been no individual year hotter than 1998, the average global temperature has continued to increase, as is evident in this plot of the NASA global temperature data:

Individuals who claim that global warming has stopped mysteriously never seem to provide any data to support it.  According to the Hadley Centre - one of the groups compiling global surface temperature data - there is no truth to this claim. 

"A simple mathematical calculation of the temperature change over the latest decade (1998-2007) alone shows a continued warming of 0.1 °C per decade."  "Global warming has not stopped."

NASA is the other main group compling global surface temperature data, and they show even more warming than Hadley (because NASA models for Arctic temperatures, where there are few temperature monitoring stations but where the warming is the greatest).

Another major problem with this myth is that it confuses short-term and long-term data.  The planet has been warming for a century, and warming most rapidly over the past 30 years.  There are short-term weather variations, and not every year is going to be warmer than the last.  However, the long-term warming trend is unmistakable.

Myth #11:  Addressing global warming is too expensive to be worthwhile

One key point this myth misses is that global climate change threatens the very future of our species (and all others on the planet).  It's hard to imagine a dollar value worth more than the future of the human race.  Even disregarding this point, the costs of addressing global warming  have been greatly exaggerated by many, as has been proven time and time again:

• One recent plan to address global warming would just cost less than 3% of the global gross domestic product (GDP) by 2030 to meet its lowest targets — or 0.12% annually. 

• The IPCC suggests similar annual mitigation costs of 0.2-3.5% of current world GDP.  That compares favorably to global economic growth that every year has averaged almost 3% since 2000. 

• The damage from unabated climate change, meanwhile, might eventually cost the global economy 5-20% of GDP each year, every year, according to a 2006 British government report.

• Moreover, Florida and California have recently performed studies regarding the economic costs of reducing greenhouse gas emissions.  Florida concluded that a 50% cut in the state's greenhouse gas emissions by the year 2025 would save the state $28 billion.  California similarly concluded the economic savings from its plan to cut greenhouse gas emissions would outweight the costs.

• An appropriate analogy is buying insurance.  A valuable object is usually worth insuring.  While this may cost a significant amount of money in the short-term, it ensures that if the object is damaged in the future, it will not be a catastrophic result.  The same is true of global warming - a significant but affordable short-term investment will help avoid potentially catastrophic damage in the long-run.  And in fact it also may even save us money.

• Still not convinced?  Consider Denmark, which Forbes named "the best country in the world for business" for 2 straight years.  Denmark has one of the strongest cap-and-trade commitments in the world — 20% below 1990 levels by 2008-2012. And it has a requirement that 20 percent of its overall energy mix be renewable by the end of 2011. And its efficiency measures are such that Energy Minister Connie Hedegaard said last year, “In 2025, (Denmark’s) total energy consumption will not have risen in 50 years.”  And it's the best country for business in the world.

• If that's not enough to convince you, consider the Congressional Budget Office (CBO) independent analysis of the Waxman-Markey carbon cap and trade bill, which concluded that by 2020 it would cost 0.2% of the average American household's after-tax income ($175 per year per average family), not including the economic benefits and other benefits of the reduction in GHG emissions and the associated slowing of climate change. 

• Similarly, the EPA concluded it would cost $80-111 per year per family per year  and consumer spending on utility bills would be roughly 7% lower in 2020 as a result of the legislation. 

• Then the EIA concluded the cost would be about $83 per average family per year with just a 3-4% increase in energy bills.

• A similar bill called the American Power Act was introduced to the US Senate by Senators Kerry and Lieberman.  The independent non-profit Peterson Institute analysis of the bill concluded "In our analysis, households see somewhere between a $136 increase and a $35 dollar decrease in annual energy expenditures, depending on future improvements in vehicle efficiency. The American Power Act also returns much of the revenue raised through the sale of pollution permits to households, with further mitigates the impact of higher energy prices."

• An analysis by the American Council for an Energy Efficient Economy estimated that the efficiency provisions alone could save businesses and consumers $22 billion annually by 2020 - the same amount the CBO concluded Waxman-Markey would cost.  In short, according to these analyses the carbon cap and trade bill will result in little to no net cost to American consumers.

• The National Association of Manufacturers (NAM) has strongly opposed the cap and trade bill, and funded a study to try and prove it would damage the US economy.  The study incorporated some unrealistically conservative and pessimistic assumptions, for example that American companies will be unable to deploy clean energy and energy efficiency technologies in a timely manner.  Neverthless, the report concluded that by 2030 if the cap and trade bill is enacted, the US GDP will grow $9 trillion, which is 95% as much as they projected it would grow if the bill fails.  They also found that 20 million new jobs would be created by 2030.  In other words, even a study with unrealistic assumptions which tried to prove the cap and trade system will cripple the economy failed to support this conclusion.

Clearly the bottom line is that reducing greenhouse gas emissions just doesn't cost that much.  The same can't be said of failing to reduce emissions.

Myth #12:  A little warming is nothing to worry about a.k.a. Warmer is Better

A frequent argument is that life has flourished during the warmer periods on Earth, therefore warmer must be better.  An obvious logical flaw to this argument is that there is a point at which the planet becomes too warm.  After all, we can't live on the Sun!

In fact, we just have to look at recent local warming events to see that warmer is clearly not better.  For example, during the European heatwave of 2003, the temperature on the continent only averaged 2.3°C warmer than the 1961-1990 average.  Yet during this heatwave, 35,000 people were killed.  There was also huge economic damage due to fires and droughts.   Wildfires in Portugal alone caused 1 billion Euros of damage. France lost 20% of its grain harvests while wheat harvests in Ukraine and Moldova were down 75 and 80% from normal, respectively. Overall, wheat production in the European Union was down 7% from the previous 5-year average.  Studies have shown that the human contribution to this heat wave was significant, and this sort of heat wave will become commonplace as the planet warms.  In fact, we're currently on pace to reach an average global temperature of 2°C warmer by mid-century.

A sub-myth is that plants will benefit from increased CO2.  While it's true that generally speaking plants do grow better when exposed to larger amounts of CO2, the associated temperature increase and subsequent increase in droughts negatively impacts their growth (as illustrated in the 2003 European heatwave).

Measurements of CO2 absorption from forests have indicated that their carbon absorption is decreasing. Ground and satellite observations have seen an increased 'browning' of the forests as well.

Scientists theorize that we may be near a turning point where forests actually become net carbon sources rather than sinks.

Myth #13:  A cold day or snowflurry in my town means global warming has stopped

A frequent misconception - particularly during the winter season - is that a cold day, cold spell, snowflurry, or other local weather event disproves global warming.

The reality is that global warming doesn't make winter weather disappear.  There are still going to be local weather variations, even record cold days, and yes, even the occasional snowflurry in Las Vegas.

If you just stop and think about it for a second, 2008 is the 9th-warmest year on record, the top 8 having been in the past decade.  So how exactly is a cold day in Las Vegas in December 2008 supposed to disprove global warming?

A sub-myth is that a hot day or heat wave or strong hurricane proves man-made global warming.  This is obviously no more true than a cold day disproving global warming.  While global warming will cause more frequent heat waves and stronger hurricanes, no single weather event can be blamed on global warming.

A second sub-myth is that snow is indicative of a particularly cold day.  The only ingredients required for snow are below-freezing temperatures and sufficient precipiation in the air.  Virtually all parts of the United States achieve below-freezing temperatures at some point during the winter.  The only difference between a snowy day and a cold day is precipitation.

Myth #14:  The U of East Anglia CRU email hack proves global warming is a conspiracy

In November 2009, the University of East Anglia Climate Research Unit server was hacked, and over 1,000 emails were stolen.  A few sentences in a few of the emails have been taken out of context and misrepresented, with various individuals claiming they expose the "global warming conspiracy".

In reality the content of the emails was quite benign and reasonable when considered in context.

In one email, scientist Keven Trenberth asks "where the heck is global warming?".  Some 'skeptics' claim this is an admittance that global warming has stopped.  In reality, as Tenberth explains, he was stating that satellite data show the amount of energy imbalance on the Earth (0.9 Watts per square meter) was not fully reflected in surface temperature data, and scientists were uncertain where the energy had gone, whether it be into melting ice, warming oceans, etc.  Trenberth states "global warming is continuing".

Another email states "It would be good to remove at least part of the 1940s blip".  Some 'skeptics' take this as an example of dishonest data manipulation.  In reality, the email is referring to an anomalous dip in ocean temperature data in the 1940s which could not be explained.  Climate scientists determined the source of the anomaly was an inconsistency in the way ocean temperature data was collected, and it appears the "blip" has now been "removed".  This was not a nefarious attempt to manipulate data, but rather an effort to explain an anomaly in the ocean temperature data.

Another email from Phil Jones states “I’ve just completed Mike’s Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) and from 1961 for Keith’s to hide the decline.”  Although when taken out of context the wording seems to reveal an intentional manipulation of data, the "trick" in question is just to plot the instrumental records along with a past temperature reconstruction so that the context of the recent rapid global warming is clear.  The "decline" which Jones it trying to "hide" is a well-known divergence problem in which certain tree ring data seems to show that global temperatures have cooled over recent decades, when in reality we know they've warmed based on surface station and satellite data.  Thus to "hide the decline" is to show the accurate instrumental temperature record instead of showing the inaccurate tree ring data over the past several decades. 

The worst of the lot, and probably the only valid complaint about the emails, are those by Phil Jones discussing how he would rather delete files than allow certain individuals to obtain them through the Freedom of Information Act.  While such an action would be completely unacceptable, no files were actually deleted, and there is no indication that Jones was doing anything more than venting his frustrations.  He didn't even delete the email where he asked other scientists to delete emails about the exchange!  Jones simply didn't want to provide his files to individuals with a history of misrepresenting climate scientists' work and dragging their names through the mud.  And lo and behold, his email was hacked, his words were misrepresented, and his name has been dragged through the mud.

One other set of emails which are often brought up are those discussing interfering with the peer-review process and preventing a paper from being included in the IPCC report.  These emails referenced a scientifically flawed paper by Soon and Baliunas, in which they made several errors like considering increased rainfall as a proxy for higher temperatures.  The paper was highly flawed and should not have passed peer review.  However, it was published by a journal which at the time was considered peer-reviewed (though as it turns out, their peer review process was extremely lax).  The journal's chief editor and several other editors subsequently quit because the journal refused to take any action in response to publishing the flawed Soon and Baliunas paper.  The journal is now Energy & Environment, which is a social science (not climate science) journal, not considered peer-reviewed, and whose editor has admitted to following her "political agenda".  Wanting to exclude a flawed paper from the IPCC report and prevent poor scientific papers from being considered "peer-reviewed" is a completely appropriate reaction to this these events.  However, the paper was referenced in the IPCC report nonetheless.

There is nothing nefarious about these emails when considered in the appropriate context, because global warming is not a conspiracy.
 

Myth #15:  The missing 'hot spot' disproves man-made global warming

According to climate models, warming at the surface of the Earth should cause an amplification effect which results in a 'hot spot' in the tropical troposphere which looks something like this:

Currently, this hot spot does not show up in the data, which has led some people to conclude that there must be a problem with climate models.

The problem with this conclusion is that the 'hot spot' should be a result of any surface warming.

When the surface and the air above it warms, the air can hold more water vapor since the vapor pressure of water increases with temperature.  Water vapor represents stored heat, so when that water vapor gets carried aloft, it moves heat upwards, warming the air at the altitude where it condenses and releases the energy contained in the latent heat of vaporization when it formed.  The tropics are where the ocean and atmosphere are warmest, so specific humidity highest, and where deep convection is largest, so that's where the tropospheric warming is greatest. 

So, regardless of how the surface warms, the upper troposphere above the tropical ocean will warm faster than the surface because of the increased upward flux of latent heat.   For example, if global warming were caused by a 2% increase in the effects of solar activity on global temperatures (solar radiative forcing), it would look like this:

Obviously quite similar.  So the fact that we're not seeing this hot spot even though both satellites and surface stations show warming at the surface doesn't mean that the models are wrong, but rather that there is a problem with the data.

The "upper air" where the hot spot should be is measured by satellites and weather balloons using instruments called radiosondes.  The US National Weather Service adjusts satellite data every week to match radiosondes, so unfortunately problems with radiosonde data may be reflected in the satellite data.  And as discussed by atmospheric physicist Steve Sherwood, there are problems with the radiosonde data:

Sherwood recently collaborated with 16 other scientists in publishing a paper comparing the various trends in atmospheric datasets with the model.   A fact sheet accompanying the paper concludes:

Another study used measurements of wind trends, which aren't subject to the same biases as weather balloon temperature measurements. As there's a direct relationship between temperature gradients and wind shear, this allows calculation of temperature trends from wind data.  The study concluded:

The bottom line with this myth is that even if the hot spot were missing in the tropical troposphere, it would not disprove the man-made global warming theory, since the hot spot should exist as a result of any warming of the Earth's surface, regardless of the cause.  Nevertheless, the missing hot spot appears to be due to problems with the data, which scientists are working on resolving.

Myth #16:  CO2 is rising because of warming oceans
 

It should first be mentioned that there is a grain of truth to this myth. As any solution warms, Henry’s Law dictates that the amount of gas that can be dissolved within it falls. So, a warmer ocean can hold less CO2 than a cooler one. This principle is the main reason for the well-known “lag” time of CO2 on past temperature records, as discussed here and here.

However, in addition to the temperature effects, the primary statement of Henry’s Law is that as the partial pressure of a gas above a solution increases, then more will be forced into the solution. So the obvious question then becomes, which effect is dominant for the current ocean-atmosphere situation? Is more CO2 being forced out of the ocean because custom essays of some natural warming process, or is more being pumped into it as a result of increasing atmospheric emissions?

As we’ll see the answer is quite clearly that oceans are absorbing much more CO2 than they are emitting, for the reasons outlined below:

Oceanic pH is decreasing

When CO2 is absorbed into a solution, it binds with a water molecule to form a molecule of carbonic acid:

CO₂ + H₂O = H₂CO₃

Although carbonic acid is technically a weak acid, it should be remembered that the ocean is a rather basic solution, with a pH of around 8.1. Because of this, H₂CO₃ has a rather strong acidifying effect in that 95% of it turns into HCO₃^-. This loss of an H⁺ ion causes the pH to decrease, which in the ocean is responsible for causing the serious and well-documented phenomenon known as ocean acidification.

If CO2 were being released by oceanic warming, then there is no good reason why pH should be decreasing. In fact, the opposite should be occurring.

Atmospheric O2 levels are decreasing

Atmospheric oxygen levels are decreasing in lock-step with CO2 emissions. This is due to the simple fact that burning carbon requires O2. When we burn an atom of carbon, the O2 that was required becomes a part of the CO2 molecule.

There's no reason that a natural release of oceanic CO2 would have any effect on O2 levels. The fact that O2 levels are falling at exactly the rate that would be expected if the CO2 emissions were being driven by fossil fuels is perhaps the clearest and simplest piece of evidence for an anthropogenic source of the CO2 increase.

Atmospheric CO2 levels are actually lower than should be expected

Calculations of land use changes and fossil fuel emissions have found that current atmospheric CO2 levels should technically be much higher than they presently are. To get a sense of scale, humans emit approximately 7 Pg annually of CO2, while oceans absorb around 2.3 Pg (another third goes into the terrestrial biosphere and the “missing sink”, leaving the final third to remain in the atmosphere).

Rise in atmospheric CO2 is consistent, while ocean temperatures are not

Temperatures have not risen smoothly, and have in fact had many erratic ups and downs over the past 100 years. If CO2 were primarily being driven by temperature, then one would expect some of this temperature variation to be seen on the CO2 records. However this is not the case, as we can see that CO2 levels rise smoothly and predictably, in accordance with the steady rise in emissions.

Oceanic CO2 is increasing fastest at the surface

Rigorous studies done by the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) have measured the amount of anthropogenic CO2 in the ocean as a result of emissions.

Among their findings, they discovered that CO2 concentrations were increasing primarily near the surface.  This is to be expected from the air-gas exchange. Since CO2 is being driven into the ocean from the air, surface concentrations are increasing much faster than deep ocean concentrations. If CO2 were being significantly released from the ocean, then one would technically expect the opposite gradient, i.e. concentrations lower at the sea surface relative to the ocean as a whole.

Isotopic signature

Though it is a bit technical, the strongest evidence in support of an anthropogenic source for the rise in emissions is the isotopic signature of the CO2—which is essentially a fancy way of saying that the carbon emitted when we burn fossil fuels is chemically different from that found in the natural cycle, such as that which is contained in the ocean.

By analyzing the carbon isotopes of atmospheric CO2, it has in fact been determined that not only are human emissions contributing to the rise in CO2, but that it has contributed 100% to the rise.

A simple explanation is that oceanic CO2 absorption and emission is unbiased to the isotope of CO2—that is, it absorbs and should technically emit just as much carbon-13 (about 1% of total CO2) as it does carbon-12 (virtually all of the remaining CO2).

On the other hand, plants—and the fossil fuels that they eventually become—do have a preference, and it is for carbon-12. Under natural variability, such as an oceanic release of CO2, one would not expect the ratio of atmospheric carbon-12 to carbon-13 to change. However, with heavy deforestation and the high combustion of fossil fuels, a disproportionate amount of carbon-12 is released.
 

 
This release has caused the ratio of carbon-13 to carbon-12 to decline since the era of the industrial revolution with a magnitude that matches the expected change from burning fossil fuels, and is on par with observations that there have been no recent significant natural CO2 releases.