Global Warming For Beginners

-- Published March 24, 2012 -- Daniel Bastian

Climate organizations around the world, and no less than arch-physicist Stephen Hawking, have communicated that climate change is the single greatest threat facing humanity today, and that the risks of ignoring this prognosis only swell with time. But somewhere between round table talks at academic conferences and the public conversation, this message has been skewed, even disfigured beyond all recognition. Whether you happen to be watching broadcast news, conversing with friends, or trying your hand at internet discussion forums, exchanges on global warming are rarely sedated affairs.

Other than the 19th centurys descent with modification, few scientific topics have been as plagued by invective and viperous reactionism. Its intersection with politics, enterprise and economics has resulted in everyone and their second cousin brandishing a hardline position on the issue like a shield in preparation for war. Though battle-tested both from within and without, the climate community has changed neither its message nor its tone. Indeed, the supporting evidence has only grown louder. Yet after a profusion of data-driven global studies, Congress appearances and public advocacy campaigns, its abundantly clear how divisive the issue remains.

Most often, objections by the non-scientist are recapitulations of rhetoric heard elsewhere, usually chaperoned by a misunderstanding of how science works or an agenda-driven distortion of the available data. Others simply betray an information deficit of the coffers of evidence backing the consensus. In this piece I respond to the most frequent objections to global warming*. While its unrealistic to cover the full spectrum of challenges, my aim is to engage the most substantive of the bunch.

*Most use the terms global warming and climate change interchangeably. While I preferentially use global warming, both labels as used today denote not only a state of monotonically increasing global temperatures, but the reverberatory effects of a mutating climate: sea level rise and coastal erosion, weather volatility and intensity, natural disasters, threatened biodiversity, fresh water availability, national security, food security and global human health.

You wont find any political or speculative rhetoric here. For example, I wont delve into the various motives one or the other side may have in furthering their respective position, or proposed metacognitive explanations for why people believe as they do. Ill intentionally eschew these ideological herrings in favor of the supremacy of data. In short, lets just sketch out what we know.

The Consensus It will first be underscored that anthropogenic global warming reflects the longstanding, highly concordant, profoundly consilient, scientific consensus (click through for all nine links). Every national scientific academy the world over finds the evidence conclusive for human-caused climate change. You can find a list of 200 of them here. Thus any murmur about controversy is confined to outside, not inside, the scientific community.

A consensus this strong can only mean that the evidence on which it is based is strong. Scientists by and large are a competitive, pedantic bunch. Science runs on disagreement, thrives even. This level of consensus is rarely reached outside of major integrating ideas like the germ theory of disease, cell theory and common descent. The unraveling of the climate denialist contingent lies with a single consideration: independent verification by different researchers using different methods and arriving at the same conclusion. The assurances of science rest upon independent lines of inquiry pursued by field-specific experts conducting stringent, cross-disciplinary studies in attempt to verify or falsify prior observations as well as make new ones. Along the way, we settle on theories that are the best fit to the evidence. Anthropogenically induced warming reflects consensus precisely because it has been independently attested by dozens of global studies using separate instrumentation, modeling and analysis. Independent attestation is the sine qua non of science, and when rhetoric has long faded, the data is what remains.

Science as a journey Science is less a list of answers than a gateway to discovery. As new data and evidence emerge, our understanding shifts if not consistent with earlier findings. To date, climate research has pointed to a uniformly clear, unambiguous conclusion: our planet is warming, at an unprecedented rate consistent with increases in fossil fuel production and mass deforestation.

Any objections to the contrary will acquire credibility to the extent they acquire empirical support. In the near future if a better explanation is put forward that holds up under the scrupulous lens of peer review, then our basket of knowledge will be overturned, its contents replaced by the newly derived information. And in the process, someone will have scored a shiny new Nobel Prize, perhaps several.

Global warming at 10,000 feet

A clear picture of global warming is best achieved with a firm grasp of two concepts: greenhouse physics and the carbon cycle. In fact, our present situation can be entirely explained by physics. The age-old question before us is how can a colorless, odorless trace gas like carbon dioxide (CO2) create such disruptive shifts in the earths ecosystems?

CO2 has been a known variable in atmospheric science for well over a century now from research by the 19th century physicist John Tyndall. The earth receives its energy from the sun. As sunlight (shortwave radiation) penetrates our atmosphere, a fraction is reflected back into space, and the remainder penetrates our atmosphere to warm the surface. The surface then re-radiates this heat in the form of infrared (longwave) radiation.

Tyndall noticed that carbon dioxide absorbs the longwave variety, blocking its escape to space. With CO2 blanketing the air, outgoing heat is trapped and re-emitted back at the surface. Consequently, Tyndall was the first to demonstrate the greenhouse effect, ostensibly named for the gases that contribute to it.1

Then in 1896, building upon the work of Tyndall, Fournier and others, the Swedish scientist Svante Arrhenius became the first to actually calculate the CO2 impact on earth temperatures. With great prescience, he predicted that combustion processes, like the burning of fossil fuels, can release enough CO2 into the air to shift global temperatures upward, all due to the greenhouse effect.2

Just as Arrhenius models indicated, when driven to extremes the greenhouse effect can interfere with the earths carbon cycle. Under natural conditions, a steady amount of carbon flows through the various reservoirs of this cycle, with a balanced exchange taking place between the biosphere, oceans and atmosphere. Prior to the Industrial Revolution, these fluxes were largely in equilibrium. But since CO2 and other greenhouses gases absorb longwave radiation in the earths atmosphere, anthropogenic activities have resulted in the earth retaining more heat than it is releasing into space. Higher and higher CO2 concentrations tip the carbon cycle into trapping more and more heat within

our troposphere, introducing a cumulative energy imbalance that gradually warms the earths atmosphere, surface and oceans.

Lets look at some objections to this theory.

Objection I For thousands of years, the earth has gone through normal cycles of temperature change. We are simply in a natural warming cycle.

A common question is how do we know the earths current warming trend is not simply part of a natural cycle? The one thing we can say with certainty about climate is that it is not static. The earths climate is in a constant state of flux and has been as far back as we can observe. The ebbs and flows of the past are well understood today. Major natural drivers include the Milankovitch cycles, which tightly describe the change in climate relative to the earths orbital patterns3; fluctuations in solar magnetic activity, which historically have correlated closely with global climate trends; and volcanism, which have periodically pumped astronomic amounts of CO2 and other gases into the atmosphere.

One of the most significant findings of paleoclimatology has been to establish, once accounting for ice age signals, that temperature lags behind CO2. That is, as atmospheric CO2 rises and falls, climate follows suit, just as the work of 19th century physicists intimated. This causative relationship has been corroborated by our oldest ice cores drilled in the East Antarctic.

Given these basic facts, we know the current period of change is irregular because the direction of change is inconsistent with causal links observed in the past and the rate of change is far greater than in historical warming periods. Stated differently, the incline in average surface-atmospheric and oceanic temperatures is much steeper in recent decades compared with the more gradual increases native to historical interglacial periods, and these trends are firmly divergent from established drivers. The level of solar intensity no longer tracks with present global climate trends as it has in the past. Oceanic warming no longer makes sense if we only include natural factors in our climate models. Volcanic, orbital and other natural forcing mechanisms simply cannot account for the sharp upshifts we are seeing. Carbon dioxide from human activities can.

To get a sense of the degree of change experienced over the last two centuries, the BEST (Berkeley Earth Surface Temperature) project charted temperature readings reaching back to the 1800s and compared them with three other major global studies. The independently conducted studies by various U.S. and European organizations, including the NOAA, NASAs GISS and the UKs HadCRUT, have all produced highly correlative data. Comprising 1.6 billion temperature records from 39,390 different weather stations across the globe, the BEST data set is by a great margin one of the best data sets ever assembled for a climate study. (The entire study is freely available online.)

Since 1880, average global temperatures have risen 0.85 Celsius (1.5 F), with about two thirds of that increase occurring since 1975. One degree of warming may not sound like much, but consider that the total variation between the peak of the last ice age and the present interglacial is 4-5 C, and that historically it has taken 5,000 years to warm those 5 degrees. Weve upped temperatures by almost 1 degree in a single century, or roughly ten times faster than the background rate of ice-age-recovery warming.

The trend is clear and shows no signs of slowing down. We are currently living in the hottest age of our observable past. Fourteen of the 15 hottest years on meteorological record have occurred since 2000, and the past three decades were the warmest on record. Historical warming periods were linked to natural causes. Our current trend cannot be adequately captured by those same linkages, which suggests an unnatural influence is culpable.

Objection II In earlier eras, there have been far higher CO2 concentrations in the earths atmosphere than today. How do we know elevated CO2 levels are human-inflicted and not natural?

While it is true that the earth has had elevated CO2 levels in past eras, we can trace the basic outlines of these events, such as when CO2 rise is preceded by supervolcano eruptions and their attendant extinction triggers or by deglaciation events activated by orbital cycles. More importantly, we can trace the temperature rise trailing the rise in CO2the true danger of GHG loading. During the Paleocene-Eocene Thermal Maximum (PETM) ~55 mya, a time when CO2 levels climbed higher than today, global temperatures rose by 5 degrees centigrade, visiting heat death upon many regions of the planet.

As of June 2014, global concentration of CO2 in the earths atmosphere sits at 398 ppmv (parts per million by volume). This chart is updated monthly and its accuracy is independently verified (theres that word again). Examining air bubbles long frozen in ice caps, like the ones recovered from the Antarctic, has revealed that the present level is higher than at any time during the last 800 thousand years and possibly the highest since the Middle Miocene some 14-10 million years ago. In other words, the last time the CO2 density was this high humans, and thus human civilization, did not exist.

But is this upswing in CO2 and corresponding temperature rise consistent with increases in fossil fuel production? Consider the following: pre-Industrial concentrations hovered around 280 ppmv, 42% lower than todays levels, and about half of that increase has been in the last three decades. In the last decade alone (2000-2009) atmospheric quantities have increased by 2.0+ ppmv each year. Zeroing in on the triggerman, when we add the observed CO2 increase in the atmosphere to the observed increase in the oceans, the sum is approximately equal to all of the coal, oil, and natural gas burned since the 19th century.

Once again, the preponderance of evidence tells us the global CO2 increase is an anomaly best explained by human-produced GHG.

Objection III Volcanoes, dying vegetation and our oceans produce far more CO2 than humans.

One of the oldest reprisals against global warming is the claim that natural CO2 emissions exceed anthropogenic emissions and therefore we cannot possibly be to blame. Unfortunately, this is utterly false. While its understood that elevated CO2 concentrations in past eras were a byproduct of heightened volcanic activity, there has been only minimal contribution across the last two centuries. Presently, volcanic blowout gives off between 130 and 230 million tonnes of carbon dioxide annually, or less than a paltry 1% of the amount released by human activities: 36 billion tonnes in 2013.

Our global reliance on fossil fuel continues to widen the gap between natural and anthropogenic CO2. In just 20 years beginning from 1990, weve upped annual CO2 from fossil fuel production from 22 billion tonnes to 33. Worse, between the years 1751 and 1900, just 45 billion tonnes of carbon dioxide were released in total, while the period between 1901 and 2008 saw a grand total of 1.2 teratonnes (1,227 GtCO2), or a 2,718% increase.4

While the burning of coal and petroleum is the leading contributor to human-produced CO2, a less discussed and far from negligible second driver is deforestation. Plants are essential to the earths carbon economy. Photosynthesizers like plants inhale CO2 from the atmosphere and exhale oxygen. When plants die, their CO2 stores are released into the atmosphere. That means every time we cut down a tree, its CO2 reservoirs are emptied, introducing more carbon gas for our ecosystems to contend with, and less vegetation to absorb it. Thus even though dying vegetation is natural, mass deforestation is not and should be addressed when a balanced planet is of utmost concern.

Tropical deforestation releases more than 1 billion tonnes of carbon dioxide each year, and the most recent estimates suggest that total annual deforestation and logging could add as much as 20% and as little as 10% more CO2 to the atmosphere.

The National Academy of Sciences now estimates that about 57% of total anthropogenic CO2 is removed via the earths natural sinks. Its clear this delicate balancing act has been stressed beyond its limits and can no longer compensate for the excess emissions were loading into the atmosphere. Again, the smoking gun lies with us, not with volcanoes, plants or other non-sentient formations.

Objection IV Carbon dioxide is a naturally occurring compound that is necessary for life to exist. CO2 means more plant growth. More CO2 is good.

Its of course true that CO2 is essential to life as we know it on earth, but it does not follow that more is better. Alcohol is not hurriedly toxic but consume twelve beers in as many minutes and youll have a newfound appreciation for the phrase drink in moderation. Similarly, steroids are naturally produced inside the body, but adding prodigious amounts on top of it can disrupt human physiology in unexpected ways. Yes, additional CO2 can stimulate plant growth, but dump too much CO2 into the atmosphere and you alter the climate to the point that those plants, and other types of life, can no longer survive. There are now over 7 billion people on this planet, with 83% of its land area now under direct human influence. It should be no surprise that we are having a marked impact.

Objection V If you look at CO2 as a percentage of the total atmosphere it only comprises .03%, much less than other gases, much too small to make any difference.

This is an interesting claim, not merely because all of the available evidence contradicts it, but because its basic premisethat there is a universal connection between concentration and effectis fundamentally flawed. After all, if you think large effects require large concentrations, youll have a hard time explaining active substances like arsenic, plutonium, hydrogen sulfide and Vitamin C. Concentrations above 300 ppm of H2S are lethal to oxygen-reliant life. So what about carbon dioxide?

When we look at the contents of our atmosphere, we see that over 99% is tied up in molecular nitrogen (N2), molecular oxygen (O2) and argon (Ar). Everything else is a trace constituent. Carbon dioxide comprises less than a mere four-hundredths of 1% of total atmospheric composition. As a fraction, it is just 1/27th of 1%. As a decimal, it is currently 0.039%.

But it is precisely the everything else that regulates the temperature of our planet. Unlike molecular nitrogen and oxygen, which are almost entirely transparent to solar and terrestrial radiation and thus have no impact on temperature, greenhouse gases are transparent to solar but opaque to terrestrial radiation. This means that everything crammed into that tiny sliver aboveCO2, water vapor (H2O), tropospheric ozone (O3), methane (CH4), nitrous oxide (N2O) and other infrared absorbersis what gives our planet its character, its ability to retain heat. Without them, we would have no greenhouse effect, the mean temperature of the surface would be 20% lower, and Earth would be one solid block of ice from pole to pole.

As you can see, it makes no sense to look at concentration without also looking at its role in the climate. While increases in any heat-trapping gas will bias the greenhouse effect toward warmer temperatures, CO2 is the most radiatively important because of its forcing potential and the feedback effects of water vapor. Pumping more CO2 into the air pushes temperatures upward, increasing the concentration of water vapor (warmer air can hold more moisture). By controlling the amount of water vaporthe biggest contributor to greenhouse warmingCO2 controls the overall size of the greenhouse effect, effectively setting the temperature for the entire planet. As weve seen, due to the proliferation of fossil fuels CO2 concentrations have skyrocketed in the last 150 years, a vanishingly brief epoch on the geologic time scale. Carbon dioxides imprint on the environment is measurable and observable and will continue in consigning us to carbon disequilibrium if left unchecked.5

Objection VI During the High Middle Ages, there was a period of time where it was actually warmer than it is today.

An important distinction to make here is that a local temperature increase does not mean a global temperature increase (and vice versa). The above claim is only true when looking at specific regions, but this tells us nothing of global temperatures. It would be like picking a handful of stock prices and inferring a trend for the overall market. During the Medieval Warm Period (950-1250 CE), warmth in some regions appears to have matched or exceeded recent levels of warmth, but a global appraisal shows that the MWP was significantly cooler than recent global temperatures.6

Its important to keep in mind that the extent of warming is not uniform across the globe, which is just what we would expect from our study of the earths ocean and wind currents. Trade winds, Hadley cells, Walker and thermohaline circulations all transport heat around the surface and ocean basins, preferencing some regions over others. This does not mean, however, that some regions are immune to global trends, only that localized trends may be less acute and will lag behind the global trend due to various thermal circulation patterns built into the earths climate system.

Objection VII I remember the summers of 19xx and 19xx being uncharacteristically cold, and people were worried about global cooling.

This one is little different from the appeal above. Cherry-picking particular regions or small sample periods is unhelpful because this fails to capture average global trends. Cold summers and warm winters fall under the domain of weather, which captures short-term variation, while climate describes long-term variation. Intermittent digressions from the broader trend (known as internal variability) are predictably normal, but over a statistically meaningful length of time underlying patterns stick out.

As for the claim that the climate community predicted global cooling in the 1970s? Thats a myth, albeit one with long legs. The majority of peer-reviewed literature during this time predicted warming from CO2 forcing. Those that didnt were based on projections in which anthropogenic sulfur dioxide (SO2) emissions, an established driver of global dimming,7 continued to rise following a spike after the Second World War.

This never happened, of course, because pollution control measures caught up to our scientific understanding of sulfate aerosols and their impact on human, non-human and environmental health; in accord with several other countries, the U.S. passed the Clean Air Act in 1963, with major amendments in the years thereafter. With regulation in place, the cooling effects of short-lived sulfate aerosols started to dissipate in the late 1970s and CO2 began to dominate the climate system once more. It is well understood today that our aerosol binge in the mid-20th century temporarily masked greenhouse warming, which resumed once those sulfates were taken out of the air.

Unfavorable Effects Here is a brief snapshot of the multiplicity of ills a warming climate brings to bear on the planet:

Ice sheets melt in response to warmer temperatures. Melting glaciers cause sea level to rise and land to submerge due to erosion and the thawing of permafrost in polar regions. Sea level rise of just 5 meters is enough to submerse major cities like New York, London and Mumbai. The Antarctic ice sheet, if melted, would raise sea level by 61.1 meters. Fueled in part by a hotter climate, weve seen a dramatic rise in natural disasters on all continents since 1980. Warm oceans are jet fuel for hurricanes, amplifying their intensity, and higher sea levels means hurricanes can penetrate further inland. A warmer atmosphere contains more water vapor, increasing precipitation levels of all storms and aggravating coastal flooding. Other links have been found between polar amplification and extreme weather events here and here. Human health is in danger due to reduced access to natural resources and water supplies, which can result in malnutrition and other health detriments.

An increase in atmospheric CO2 is directly linked to increasing acidity of the oceans. This poses a serious extinction threat to major classes of marine organisms, including corals and bivalves, which will have ripple effects all their own. (Also see study here.) Increasing temperatures affect plant life by disrupting food chains and reducing botanical biodiversity. As with plants, hotter climates impact the animal kingdom in the form of sabotaged food chains, extinction of species and the erosion of genetic diversity. For example, longer summers and fast-receding sea ice impact polar bear populations as they depend on intact sea ice for their primary food sourceseals. Environmentally induced migration puts bird species at risk of extinction; see here and here. Seasonal disruptions: Earlier spring and later frost means continually higher pollen counts for more of the year, increasing allergic reactions among humans especially. Condensed winters means fewer insects die, resulting in more to contend with in the warmer seasons. Higher CO2 concentrations means more potent poison ivy.

There are also a number of feedback loops operative in a warming planet. When sea ice melts, the earth becomes darker as a result of the lower reflectivity of sea water relative to sea ice. Otherwise known as the ice-albedo feedback, this causes more sunlight to be absorbed, warming the oceans at an ever faster rate. Warmer oceans then melt more sea ice, reducing planetary albedo even further. More catastrophic is that warmer oceans melt ice shelves from underneath (called basal melt), and the thawing of ice shelves affects the flow of adjacent, non-floating ice (i.e., ice sheets and glaciers), thereby contributing to sea level rise. Warmer oceans also emit more CO2, causing more warming and increasingly active weather. The link in the chain that demands to be severed is human carbonization.

Conclusion Its impossible to talk too much about global warming. The evidence for it couldnt be less opaque: our escalating attachment to fossil fuels and greenhouse gas emissions has provoked feedback effects of reciprocal degree and disrupted the Earths energy budget. The climate research community has delivered this message unambiguously for more than two decades now, emphasizing in turn that chances are superlative that if we fail to act timely and sufficiently the repercussions will be enormousto our way of life and to those with which we share a natural habitat. Every living organism and every ecosystem are influenced in some tangible way by the perturbations weve enacted. As custodians of this vibrant planet and the most observant of species, is it not our responsibility to shade our treasured kin from the consequences of our actions? If we are sensitive to the realities of climate science, humanity may one day worthily bear the title of Lifes Protectress. But there is much to do. We must move past the emotionally charged and politically fabricated debate and work together to hammer out feasible solutions for arresting climate change and tempering its ill effects. I hope this piece answers some of the more frustrating questions out there, and more importantly I hope it helps disentangle preconceived notions and sweep away internal appraisals, leaving nothing but the science behind.

I think we understand the mechanisms of CO2 and climate better than we do of what causes lung cancer. In fact, it is fair to say that global warming may be the most carefully and fully studied scientific topic in human history.

Ralph Cicerone, President of the National Academy of Sciences

For newcomers, The Guardian has compiled an excellent FAQ flowchart, and The Royal Society has prepared a concise primer.

Those interested in burrowing deeper are encouraged to pick up the following recommendations: The Rough Guide to Climate Change, 3rd edition (2011)

Storms of My Grandchildren: The Truth About the Climate Catastrophe and Our Last Chance to Save Humanity (2010) The Hockey Stick and the Climate Wars: Dispatches from the Front Lines (2012) Funny Weather: Everything You Didnt Want to Know About Climate Change But Probably Should Find Out (2006) Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming (2010)

Footnotes:

1. Air Force and Navy pilots noticed first-hand this relationship between atmospheric CO2 and infrared energy when they found that high CO2 concentrations obstructed certain heat signatures on their radar systems. When using on-board heat sensors designed to pick up wavelengths in which CO2 is active, the visibility of signatures was severely compromised due to CO2 absorption. The Air Force still use these spectroscopic databases derived from calculations for heat-seeking missiles today, and its but one in a long line of validations of what Tyndall discovered so long ago. You can find a collection of Tyndalls original research here. []

2. From his 1906 work Vrldarnas utveckling (Worlds in the Making):

Any doubling in the percentage of carbon dioxide in the air would raise the temperature of the earths surface by 4 degrees; and if the carbon dioxide were increased fourfold, the temperature would rise by 8 degrees. []

3. There are three main types of orbital cycles that can impact climate: eccentricity, obliquity (axial tilt) and precession. Each of these changes has characteristic periodicities. Eccentricity varies on an approximately 100,000 year cycle, obliquity on a nearly 41,000 year cycle, and precession over an approximately ~22-26,000 year cycle. Currently, the earth is closest to the sun (perihelion) just about at the peak of southern hemisphere summer, northern hemisphere winter. []

4. I downloaded the Excel file from CDIAC, compared the periods of 1751-1900 and 1901-2008, then multiplied the totals by 3.667 to convert the atomic mass of carbon to the molecular mass of carbon dioxide as specified here. Units are in metric tons (tonnes). Note that these figures capture total carbon emissions from fossil fuels (including gas, liquid and solid fuel consumption, cement production, and gas flaring) and does not include emissions from land-use changes. []

5. While CO2 is the major offender and a permanent fixture of any conversation about climate change, there are several other forcing agents that influence terrestrial temperatures. As noted, water vapor is massively important due to its amplification effects of other greenhouse gases. Other radiatively active gases, like methane and nitrous oxide, actually have higher global warming potential (GWP) than carbon dioxide when considered over shorter time scales.

For example, a given quantity of methane has 84 times the GWP of CO2 over a 20-year period and 28 times over a 100-year period. Considered over the same 100-year period, nitrous oxide has 298 times more impact per unit mass (GWP) than CO2. [Source] While both of these gases trap more heat than CO2 over their atmospheric lifetimes, CO2 has much longer residence times due to its role in the carbon and other biogeochemical cycles.

Over half of the CO2 released into the atmosphere is removed from the air on a time scale of 30-95 years as its mixed into the upper oceans (30-40%) and taken up by plants and other photoautotrophs during photosynthesis. This is the fast phase of the carbon cycle. However, around 20% of emitted CO2 lingers for many thousands of years for reasons related to equilibration between the deep oceans and atmosphere. It is these slower phases (including an even longer tail involving the geological reservoirs of the earth system) that make CO2 so potent. As David Archer explains, different parts of the ocean equilibrate with the atmosphere on different time scales, ranging from a year for the tropical surface ocean to a millennium for the deep sea. So unlike the exponential decay of other GHGs, CO2 persists on much longer time scales as it loops through the earths natural processes. Ultimately what this means is that even if we were to cease CO2 emissions tomorrow, global concentrations would not return to pre-Industrial levels for more than a thousand years.

Lastly, aerosols and clouds are also important components of the climate system, with both direct and indirect effects on climate. Unlike greenhouse gases, which are positive forcing agents, these are generally negative forcing agents as they remove heat from the climate equation. One notable exception is black carbon or soot. Black carbon particles are a byproduct of incomplete fossil fuel combustion that contribute to global warming by reducing albedo (the ability to reflect sunlight) when deposited on snow and ice. Many scientists now believe it is the second biggest contributor to global warming behind CO2. []

6. From the IPCC Fourth Assessment Report: Regionally restricted evidence by itself, especially when the dating is imprecise, is of little practical relevance to the question of whether climate in medieval times was globally as warm or warmer than today. []

7. Global dimming refers to the reduction of solar radiation received at the surface. Sulfate particulates and other aerosols clogging the upper atmosphere reflect solar energy back into space, preventing it from ever reaching the planet surface. Thus, SO2 emission acts as a negative forcing agent as it removes heat from the climate system, in contrast to CO2 which acts as a positive forcing agent. []

Contact: Daniel Bastian

Website: Waiving Entropy