Not only do people wonder why climate change is a problem when it "happened in the past" (see below), but they question the existence of any evidence. Yet it has been accessible for many years (examples here, here, and here). We have a rise in the decadal averages in three major surface datasets (affirmed by the tropospheric satellite record), borehole analysis, global ocean heat content (erroneous claims of cooling aside), widespread glacier disintegration, rapid sea ice "rot" (excepting Antarctica's special case), thousands of bio-markers, increasing ratios of record high to record low temperatures, a rising tropopause, increasing water vapor as temperatures rise, thawing permafrost... All combined with the well-established infrared absorption properties of CO2/CH4, and a reduction of infrared energy emitted from Earth. There's a distinct human fingerprint in the climate of the last several decades, despite the temporary offsetting effect of things like sulfur pollution and natural variability.

Science has moved beyond just strengthening the attribution case, to refining projections based on physical processes (linear and otherwise), paleoclimatology, and trends in human activity. Uncertainty remains on a number of details, partly because there's no exact prehistoric analog to our situation (a relatively rapid, globally-distributed forcing coupled with a black carbon influence, and mixed with the short-lived cooling effect of sulfates). But the main conclusions are solid, and the stakes are high for the most populous civilization to exist on Earth. There is, though, a disconnect between the mainstream scientific community and the controversy-driven media. That would include claims that global warming doesn't affect weather (when over time it can't help but do so), and of it "stopping" every time there's a short down-tick in surface temperature, vs. a statistically significant trend visible even on smaller timescales:

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What the scientific community is saying (reflecting the consensus among actual published researchers):

A strong, credible body of scientific evidence shows that climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems...
- National Academy of Sciences

The bottom line is that CO2 is absolutely, positively, and without question, the single most important greenhouse gas in the atmosphere. It acts very much like a control knob that determines the overall strength of the Earth’s greenhouse effect...
- Dr. Andrew Lacis, NASA climatologist

To slow the rate of climate change, we can decrease the amount of carbon dioxide that we release into the atmosphere.
- National Center for Atmospheric Research

There is no single threshold above which climate change is dangerous and below which it is safe. There is a spectrum of impacts. But some of the largest impacts are effectively irreversible and the thresholds for them are very near... In particular, the melting and breakdown of polar ice sheets seems to be in the vicinity of a couple of degrees warming. This expectation is based on current high rates of mass loss from the ice sheets compared to relative stability through the Holocene (the past 10,000 years) and on past ice sheet response in periods such as the Pliocene (a few million years ago) when the Earth was a couple of degrees warmer than preindustrial times (and sea level up to 25m higher)...
- Dr. James Risbey, CSIRO Australia

Recent observations confirm that, given high rates of observed emissions, the worst-case IPCC scenario trajectories (or even worse) are being realized. For many key parameters, the climate system is already moving beyond the patterns of natural variability within which our society and economy have developed and thrived. These parameters include global mean surface temperature, sea-level rise, ocean and ice sheet dynamics, ocean acidification, and extreme climatic events. There is a significant risk that many of the trends will accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts. 
- 2009 Copenhagen climate congress of 2,500 scientists

The Earth's climate is now clearly out of balance and is warming. Many components of the climate system—including the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasons—are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century...
- American Geophysical Union

There is no doubt that Greenland ice loss has not just increased above past decades, but it has accelerated. The implication is that sea level rise estimates will again need to be revised upward.
- Dr. Jason Box, Glaciologist

It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there’s always an element of both.
- Dr. Kevin Trenberth, NCAR

A small percentage of the atmosphere, CO2 is nevertheless the primary persistent "greenhouse gas", re-radiating heat energy over an atmospheric lifetime of several years. But a significant accumulation (total volume in the atmospheric column being what's important), exceeding the carbon cycle's quasi-equilibrium, can last centuries and subside over millennia. After all, CO2 molecules aren't just absorbed in nature, they're also re-released. Carbon is constantly exchanged between the oceans, atmosphere, and biosphere, so seemingly modest atmospheric "residence times" are misleading. The net uptake resulting from a pressured carbon cycle would slowly reduce the total "pool" once emissions fall (assuming no big feedbacks beforehand). But like an overflowing bathtub with a slow drain, the input must drop below the sink rate. Then, even with no biological sink reduction, carbon transfer rates to the deep ocean are an absorption "bottleneck". So barring fantastical future advancements in carbon sequestration, today's buildup is a very long term commitment.

Carbon dioxide plays vital roles in climate and the biosphere, but there can be too much of a good thing. A buildup amplifies the greenhouse effect that keeps Earth's average temperature above zero, causing warming (2). Research overwhelmingly indicates this is the main factor in a climatically strong trend, with much of the extra heat absorbed and distributed by the oceans (their thermal inertia produces a lagged atmospheric response). This rise, subject to fluctuation from things like ocean cycles (in the exchange of heat between the depths and the surface) and sulfate "aerosol", is extensive but not uniform, and proxy studies suggest it has already exceeded anything in at least 2,000 years.

Heat is the ultimate driver of the climate system. Effects on evaporation, all forms of precipitation, reflective ice cover, oceanic and atmospheric circulation, and storm intensity make "global climate change" a more complete descriptor of the situation. Although it's still early in the process, and there are variables between climate change and disaster losses that so far make direct attribution between them difficult, trends in heat waves and precipitation are already clear. And along with fast responses like increasing levels of water vapor (a reactive greenhouse gas), protracted warming is also subject to amplification by an interplay of longer-term feedbacks, like reduced carbon storage (example1, 2, and 3), and the release of more greenhouse gas from warming oceans, forests, peatlands, and tundras. Higher wildfire incidence and increased CO2 and methane emission from thawing permafrost (See sidebar) are examples. How fast carbon cycle effects will be is uncertain, but even the substantial mid-range climate projections have insufficiently assessed their potential, and with improved understanding of amplifying feedbacks and their lasting dominance, concerns have only grown.

Warming may not seem like an urgent problem in most temperate regions, where so far it has been largely subtle and mixed with significant variability. But it does have the potential to cause lasting disruption, since many things in today's world are vulnerable to rapid change. The rate of warming is a key to how ecosystems and large human populations will be impacted. Many a scientist would say past climate change has helped shape humanity (usually in large-scale transitions over thousands or millions of years), but so has the relatively mild, stable holocene in which intensive agriculture and modern civilization have developed, allowing us to expand our horizons beyond the mere struggle for survival.

Nature is not without resilience, but with cumulative pressures, biomes can weaken and change can snowball. Yet we can limit it's progression and protect the biologically-rich interglacial that has helped societies thrive. Markets are more globalized, so this will take international efforts to reduce emissions from sources like electric generation, animal agriculture, and transport (none of which are likely to realize sufficient cuts alone). As well as technology to handle changes underway. And those in the middle and upper classes, who've benefited most from a fossil-fueled system, are the ones more likely to have the means to foster change. Still, certain economic and ideological forces seek to cast doubt on strong science, or downplay the negatives while emphasizing some regional benefit (from moderate warming). An industry of disinformation has arisen, similar to past efforts to deny the health effects of cigarettes, with some of the same players (below). And those PR operations have helped delay a stewardship-oriented approach.

Delay means locking in stronger impacts, including on agriculture and resource reliability. This issue is about rapid climate change, and its many effects on today's biosphere. If we stop ourselves from pushing too far, we at least have the chance at several thousand years of advancement; to become more resilient as a civilization, not just as a species.

Global warming refers to global average temperature change over time (an indicator of Earth's "energy budget": Energy in vs. energy radiated to space). It doesn't mean constant, regionally synchronous change or the end of cold snaps. Despite the trend, fluctuations continue in a fluid-dynamic system, particularly from the variability in ocean heat exchange and heat distribution . These maps give some idea of how anomalies can vary on short timescales, especially regionally:


Record highs will also occur more frequently with global warming (already observed, with record highs occurring about twice as often as record lows over the past decade), but local extremes alone are not necessarily global indicators. It can be interesting, though, to consider the circumstances under which records are set. Frequently anomalous warmth despite the cooling influence of la niña, increased Asian emissions of sulfur particulates, and a deep solar minimum could be viewed differently from past warm spells under the opposite conditions.

An example of a trend in record temperature ratios, expected to accelerate:

Robert W. Felix, a former architect, received some publicity for his book and website claiming that we're actually entering an ice age, and that glaciers are growing (more below). These claims were repeated by botanist David Bellamy, and posted on "skeptic" sites like Steve Milloy's "junkscience.com" (more). The assertions were traced to figures published by Fred Singer (who, like Milloy, was connected with TASSC, and also with dubious petition projects, below). Singer stated his source as "A paper published in 'Science' in 1989" - nowhere to be found.

It has also been suggested that we're averting another ice age by tipping the climate scale with greenhouse gas emissions. Although we've probably already tipped that scale enough to significantly delay a Milankovitch-type glaciation, indications are that Earth is nowhere near due for another glacial period anyway. Humanity has been blessed with one of the longer interglacials. Still, there are those who have used the ice age aversion argument as positive spin. They're essentially implying that rapid holocene warming is fine because we'll avert the subtle multi-millennial cooling (or very rare, 'well-timed' supervolcanic eruption) leading to the next big freeze. But if we're still around, we can consider measured greenhouse gas emission/other geo-engineering to avoid glaciation. For now, humans are in a period of development that may eventually make us less vulnerable to disaster. It's in our interest to value and prolong the time we have.

On a related issue, there's a common myth that climatologists predicted an imminent ice age in the 1970's, despite the infancy of the science & technology, and acknowledged uncertainty on the future trajectory of climate forcings. While there was some speculation (along with caveats) and some overzealous media coverage, there's more to the story than today's naysayers admit. Update: A more recent review paper affirms what the scientific literature was actually saying.

For those with a Bible-based perspective, the question becomes whether there's direct divine control over Earth's life-supporting systems (thus we can freely trash them), or whether a sustained quasi-equilibrium has been set up that allows our actions (born out of free will) to demonstrate either stewardship or disregard. And which seems more responsible: To assume the former because it's easy and convenient, or the latter because it's not worth gambling with the future (a future that could be millennia, despite ever-present predictions of apocalypse)? Several religious groups have decided it's better to err on the side of caution and accept some responsibility for the environment that sustains us. Others selectively interpret the Bible as supporting careless plunder, while disregarding passages that suggest such things as humanity's self-determination.

12• "More solar activity is responsible, and Mars/Pluto prove it.

Some highlight the sun as the primary cause of global warming (often selectively citing snippets from Solanki et al. 2004), but the science doesn't support this. Although the sun goes through 11-year activity cycles, and contributed to moderate (but regionally significant) warming in the early 20th century, it doesn't account for the strong 'trend' of recent decades. A 2006 NCAR study (Foukal et al.) affirmed with satellite and proxy data that solar flux has made a negligible contribution to accelerated warming over at least the past 30 years, and that luminosity changes had a moderate influence over the past thousand years or more. Lockwood & Frolich note in another study that "over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures". Other solar-related claims have also taken it on the chin. Additionally, the stratosphere has been cooling (#24), consistent with an amplified greenhouse effect but not with a significant increase in average solar output.

The question is, what's causing that ocean warming if solar activity can't account for most of it? A few "skeptics" have suggested undersea volcanoes, but a major activity increase (for which there's no evidence) would be required to globally heat the oceans a fraction of a degree, and they're also warming from the surface down. Earth's average geothermal heat flow is negligible compared to the energy received from the sun or retained by the greenhouse effect. A recent Scripps study (see sidebar) affirms that patterns of oceanic warming are consistent with CO2 forcing (as are other changes). And in terms of CO2 exchange, the oceans still represent an absorption of about 2 GtC (subject to decline as temperatures rise and ocean chemistry changes). Even fossil fuel accounting, and the fact that part of our carbon output doesn't remain in the atmosphere, indicates that nature remains (at least for now), a net carbon sink, not a source. Scientists can also measure how much CO2 is from fossil fuels:

How do we know that recent CO² increases are due to human activities?

This claim not only ignores the traceable origins of the current accumulation (above), but it leaves out some important details. During glacial period terminations, recovery of atmospheric CO2 acted as a feedback to amplify warming triggered by Milankovitch orbital forcing. This doesn't mean CO2 can't itself be a significant climate forcing (a cause of change rather than just a response). It just never has (until now) over the entire 800,000 year ice core record. More from Realclimate.

Jeff Severinghaus, Professor of Geosciences at Scripps notes:  "All that the lag shows is that CO² did not cause the first 800 years of warming, out of the 5000 year trend. The other 4200 years of warming could in fact have been caused by CO², as far as we can tell from this ice core data".

According to Caillon et al, 2003, "the CO² increase clearly precedes the Northern Hemisphere deglaciation"...

And referring again to our 650 kiloyear CO2 chart, we can see the other difference between then and now:

A study by Lyman et al showed a decrease in oceanic heat content from 2003-2005. Such variability wouldn't necessarily indicate a new trend.  But there also seems to be an inconsistency in this study: Sea level should have dropped along with temperature, unless there's been a sharp increase in compensating ice melt. Ocean circulation changes and variability in heat exchange with the deeper ocean are variability factors. While the study is still under review, short-term inconsistency in warming wouldn't be surprising.

Update: Peer review at work. Lyman study seems to have a data problem, cooling has disappeared in latest analysis. And some discussion here and here of trends and updates since.

Although it's emissions and concentrations are much lower than those of CO2, methane is a more powerful greenhouse gas, and has the potential to play a significant role in future warming. Along with the again rising levels of directly human-induced emission, feedbacks are likely to release more methane as warming progresses. One scenario, modeled by an NCAR supercomputer, involves warmer water releasing methane currently frozen under the sea floor (see sidebar). Thawing permafrost, though, may have more feedback potential over the next several decades.

CO2 out-gassing from volcanism may have been a significant climatic influence in prehistory, but more "recently" in geologic time, it has been dwarfed by human output (currently about 30 gigatons total mass/8 gigatons carbon equivalent). Gavin Schmidt notes that there are both direct volcanic emission measurements (see here), "... and also isotopic and mass balance arguments that absolutely, 100%, no question about it, mean that the current rise in CO2 is anthropogenic". Even supervolcanic eruptions in prehistory have had much less impact on atmospheric concentrations than the sustained human output.

Another volcanic gas, sulfur dioxide (SO²), can have a temporary cooling effect.

But the real issue is a significant change in CO2 concentration. Overall, natural CO² emissions have been balanced by absorption. A simplified example: The trees that burn in a forest fire contain carbon that was previously removed from the atmosphere. As the forest regenerates, CO² is re-absorbed. With continued warming, though, wildfires are expected to become more frequent and severe, further disrupting the carbon cycle. Update: Rise in wildfires linked to climate change.

Readings are taken upwind from volcanic vents, out-gassing is monitored, and any short-term spikes are flagged for removal, but even if the Mauna Loa figures were tainted, the volcano can't explain a long-term upward trend. There are also multiple isolated measurements around the world, including in Antarctica, that agree well. More here on the choice of Mauna Loa, and here on measurement reliability/verification.

It has been argued by certain fossil fuel interests that higher CO² levels will enhance plant growth and benefit agriculture. This is true to a point, under the right conditions. But in the real world, plant growth and productivity are more limited by factors other than CO2 concentration (which is already quite sufficient to support healthy growth rates). Growing conditions are likely to change (insect populations, water availability, extreme weather events...), and a comprehensive Stanford University study found that elevated CO² only stimulated growth when nitrogen, water and temperature were at normal levels. It was also suggested that excess soil carbon or initially accelerated growth may limit the availability of nutrients. Some "weedy" plants may benefit disproportionately from extra CO2, but they would need to somehow overcome present-day geographic limitations and more than triple the net terrestrial carbon sink to counter most of our current emissions. Any enhancement of a sink helps, but the accumulation already in progress is exceeding sequestration by plants, and oceanic dissolution. For more detail on considerations in agriculture, see the consequences page.

Some contrarians have suggested that extra cloud cover might offset warming. John Christy of UAH speculated on this, and also stated (to digress for a moment) that "Whatever happens, we'll adapt". The question is how many of Earth's billions of people (ecology aside) might adapt, at what cost, and with what quality of life? Regarding clouds, though: Although storm intensity may increase, rising temperatures also mean more water can remain uncondensed in the troposphere. The likelihood of significant cooling from thick, low-level cloud cover appears questionable. Additionally, proponents of the cloud-cooling theory don't seem to indicate why negative feedback had trouble keeping up with the last carbon accumulation event in prehistory, or even how Earth has come out of glacial periods despite a supposedly strong negative feedback. Betting on altered cloud cover is probably unwise.

Depending on their type, clouds can reflect more energy than they trap in the atmosphere or vice-versa. They can reduce daytime warmth, but also keep overnight temperatures higher and provide a head-start for daytime heating. An overall increase in winter lower-level cloud cover and a reduction in the summer averages would have a warming influence. The net effect is an area of intensive study. All climate models project significant warming, but cloud behavior may help determine whether it will be closer to the high end or the low end of the range. Update: New research suggests the higher end.

Spencer & Christy, previously known for promoting a flawed satellite temperature record,  released a paper suggesting that an inter-annual cooling effect in the tropics may have implications for the projection of global warming. See How to cook a graph in three easy lessons.

These are an oscillation related to the circulation of tropical Pacific heat. The global average temperature impact of a strong El Niño is about 0.2 degrees C. Although a significant natural influence on weather patterns and contributor to inter-annual fluctuation (see graph at the top of this page), the temperature effects of the natural cycle smooth out over years, and aren't a cause of the overall trend. However, the effects of El Niño may be "enhanced" by global warming, since the oscillation essentially pools oceanic heat and alters moisture flow. Recent NASA and NOAA research suggests that much of the increase in Pacific sea surface temperatures in recent decades has so far manifested itself in the form of stronger, more frequent central Pacific El Niños. Discussions of how the average magnitude and persistence of El Nino may be affected here and here.

Related:
Natural ENSO responsible for warming trend?
"Atrocious" paper makes it into JGR

Although sometimes confused in the media, stratospheric ozone depletion and global warming are largely separate concerns. The main problem with ozone depletion is higher levels of damaging ultraviolet radiation. This is already being successfully tackled (see here for more, including footnotes) with an international phase-out of CFCs (ChloroFluoroCarbons), while global warming needs to be addressed via reduced emission of greenhouse gases ("GHGs") like carbon dioxide. The two issues, though, are somewhat interconnected.

CFCs (and some of their substitutes) and tropospheric ozone are also greenhouse gases (relatively minor ones compared to CO2). And, as counter-intuitive as it may seem, the amplified greenhouse effect causes stratospheric cooling (simplifying, more GHG in the troposphere reduces infrared re-radiation to the stratosphere). Ozone depletion itself contributes modestly to such cooling.

The claim that band saturation severely limits CO2 as a climate forcing is a myth (more here and here, and this on the claims of Ferenc Miscolczi), derived from a concept widely considered flawed since the 1950's. The upper atmosphere and the "wings" of the CO2 absorption band are rather important, and models already represent CO2 forcing as the natural log of it's change in concentration (leaving plenty of climate-shifting potential).

These are popular half-truths, meant to mislead. Partly due to heat uptake by the southern ocean and lower surface melt, ice sheets in East Antarctica have remained relatively stable (consistent with model projections), and appear to have received some extra snowfall. Some snowpack thickening has also been observed in Greenland at high elevations. This is related to the regional precipitation of extra moisture, and is not inconsistent with a warming world. Despite this, and a slight cooling in parts of Antarctica, most of the world's glaciers have been receding as part of an inter-decadal trend, and ice loss has accelerated in Greenland (resulting in a large net mass reduction).

On Antarctica, new research shows a significant net loss of land ice (while the growth of Antarctic sea ice has been attributed to a complex interaction of strengthened cyclonic winds, and changes in precipitation and ocean circulation).  Update: Antarctic Loss Speeds Up, Nearly Matches Greenland. Update2: Ice sheet-ocean margins more vulnerable, important than realized.

The connection between recent hurricane strength and early-stage global warming has been an area of some debate. The link with hurricane frequency is even more so. It's typically not possible to prove a direct causal link with a particular event, since such events are a confluence of regional dynamics and the overall trends related to Earth's energy balance. Ocean warming simply makes more energy and moisture available for potential concentration, thereby increasing the odds of a severe event. Research (such as Emanuel, Knutson, Webster, Holland et al.) indicates an intensity trend influenced by warming. As warmer temperatures spread north, tropical cyclones can also stay intact longer.

Even a seemingly small rise in sea surface temperature means extra water vapor and energy for a storm to pick up (while el niño or Saharan air layer conditions can periodically suppress hurricanes in the Atlantic). Then there is the element of rising sea levels and their contribution to storm surge. Although the IPCC estimates are often cited, those carried substantial caveats regarding dynamic ice sheet disintegration, and the lack of thoroughly reviewed projections for it as of the AR4. More recent research suggests we could see a foot of sea level rise (on average) by 2050 and 4-6 feet by 2100. And some of the newest, preliminary research is exploring the possibility that high pressure blocking ridges in a warming Arctic can influence the path of a storm like Sandy, so it's less likely to drift out to open sea.

Still, certain meteorologists and media outlets have quickly dismissed any climate change connection, ignoring the ongoing scientific inquiry, often focusing on frequency rather than intensity, and chalking everything up to a natural Atlantic cycle (controversy followed). The research, though, found an intensity trend in Pacific and Indian ocean storms as well. This earlier article addresses the issue further.

Although consensus isn't the point of science, and some might mock it, there have been relatively few cases of established scientific tenets being fundamentally overturned in modern Earth science. For every Galileo-type challenge that succeeds in establishing an alternative theory, there are many others that fail. In climatology, the peer-reviewed literature and periodic assessments, and major scientific organizations indicate wide agreement on several main points, including that there's a significant human influence on climate:

As Ray Pierrehumbert put it: "Money and perks! Hahahaha. How in the world did I miss out on those when I was a lead author for the Third Assessment report? Working on IPCC is a major drain on ones' time, and probably detracts from getting out papers that would help to get grants (not that we make money off of grants either, since those of us at national labs and universities are not paid salary out of grants for the most part). We do it because it's work that has to be done. It's grueling and demanding, and not that much fun, and I can assure everybody that there is no remuneration involved..." And "...scientists are probing theories and conceptions all the time, trying to break them. The best way to become famous is to overturn established wisdom, so scientists look hard all the time for opportunities to do this."

Further, much attention is applied to areas of uncertainty - something climatologists concerned mainly about money would over-emphasize, rather than affirming the primary role of human activity. We also have the reality that many climate researchers are tenured/conduct research as they see fit, and that funding goes into cutting edge research and expensive equipment. Can the same be said of funds disbursed to contrarians by the fossil fuel industry? Even the finding of a significant natural factor would attract research grants. And there have been decades of opportunity for any number of skeptical scientists to overturn the case for anthropogenic influence. Instead it has grown stronger.