Science says: Climate reacts to whatever forces it to change; humans are now the dominant force.
A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. To understand this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it's forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.
There are a number of different forces which can influence the Earth's climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.
It is obviously true that past climate change was caused by natural forcings. However, to argue that this means we can't cause climate change is like arguing that humans can't start bushfires because in the past they've happened naturally. Greenhouse gas increases have caused climate change many times in Earth's history, and we are now adding greenhouse gases to the atmosphere at a increasingly rapid rate.
Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, for instance, we can work out the degree of past temperature change, the level of solar activity, and the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods and timescales in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat. We can even quantify this: when you include positive feedbacks, a doubling of CO2 causes a warming of around 3°C.
What does that mean for today? Rising greenhouse gas levels are an external forcing, which has caused climate changes many times in Earth's history. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the greenhouse warming. So past climate change doesn't tell us that humans can't influence climate; on the contrary, it tells us that climate is highly sensitive to the greenhouse warming we're now causing.
Science says: In the last 35 years of global warming, sun and climate have been going in opposite directions.
Over the last 30 years of global warming, the sun has shown a slight cooling trend. Sun and climate are going in opposite directions. This has led a number of scientists independently concluding that the sun cannot be the cause of recent global warming.
One of the most common and persistent climate myths is that the sun is the cause. This argument is made by cherry picking the data - showing past periods when sun and climate move together but ignoring the last few decades when the two diverge.
Science says: Negative impacts of global warming on agriculture, health, and environment far outweigh any positives.
While CO2 is essential for plant growth, all agriculture depends also on steady water supplies, and climate change is likely to disrupt those supplies through floods and droughts. It has been suggested that higher latitudes—Siberia, for example—may become productive due to global warming, but the soil in Arctic and bordering territories is very poor, and the amount of sunlight reaching the ground in summer will not change because it is governed by the tilt of the earth. Agriculture can also be disrupted by wildfires and changes in seasonal periodicity, which is already taking place, and changes to grasslands and water supplies could impact grazing and welfare of domestic livestock. Increased warming may also have a greater effect on countries whose climate is already near or at a temperature limit over which yields reduce or crops fail—in the tropics or sub-Sahara, for example.
Warmer winters would mean fewer deaths, particularly among vulnerable groups like the aged. However, the same groups are also vulnerable to additional heat, and deaths attributable to heat waves are expected to be approximately five times as great as winter deaths prevented. It is widely believed that warmer climes will encourage migration of disease-bearing insects like mosquitoes and malaria is already appearing in places it hasn't been seen before.
While the opening of a year-round ice free Arctic passage between the Atlantic and Pacific oceans would confer some commercial benefits, these are considerably outweighed by the negatives. Detrimental effects include loss of polar bear habitat and increased mobile ice hazards to shipping. The loss of ice albedo (the reflection of heat), causing the ocean to absorb more heat, is also a positive feedback; the warming waters increase glacier and Greenland ice cap melt, as well as raising the temperature of Arctic tundra, which then releases methane, a very potent greenhouse gas (methane is also released from the sea-bed, where it is trapped in ice-crystals called clathrates). Melting of the Antarctic ice shelves is predicted to add further to sea-level rise with no benefits accruing.
A cause for considerable concern, there appear to be no benefits to the change in pH of the oceans. This process is caused by additional CO2 being absorbed in the water, and may have severe destabilizing effects on the entire oceanic food-chain.
The effects of glaciers melting are largely detrimental, the principle impact being that many millions of people (one-sixth of the world's population) depend on fresh water supplied each year by natural spring melt and regrowth cycles and those water supplies—drinking water, agriculture—may fail.
Sea Level Rise
Many parts of the world are low-lying and will be severely affected by modest sea rises. Rice paddies are being inundated with salt water, which destroys the crops. Seawater is contaminating rivers as it mixes with fresh water further upstream, and aquifers are becoming polluted. Given that the IPCC did not include melt-water from the Greenland and Antarctic ice-caps due to uncertainties at that time, estimates of sea-level rise are feared to considerably underestimate the scale of the problem. There are no proposed benefits to sea-level rise.
Positive effects of climate change may include greener rain forests and enhanced plant growth in the Amazon, increased vegetation in northern latitudes and possible increases in plankton biomass in some parts of the ocean. Negative responses may include further growth of oxygen poor ocean zones, contamination or exhaustion of fresh water, increased incidence of natural fires, extensive vegetation die-off due to droughts, increased risk of coral extinction, decline in global photo-plankton, changes in migration patterns of birds and animals, changes in seasonal periodicity, disruption to food chains and species loss.
The economic impacts of climate change may be catastrophic, while there have been very few benefits projected at all. The Stern report made clear the overall pattern of economic distress, and while the specific numbers may be contested, the costs of climate change were far in excess of the costs of preventing it. Certain scenarios projected in the IPCC AR4 report would witness massive migration as low-lying countries were flooded. Disruptions to global trade, transport, energy supplies and labour markets, banking and finance, investment and insurance, would all wreak havoc on the stability of both developed and developing nations. Markets would endure increased volatility and institutional investors such as pension funds and insurance companies would experience considerable difficulty.
Developing countries, some of which are already embroiled in military conflict, may be drawn into larger and more protracted disputes over water, energy supplies or food, all of which may disrupt economic growth at a time when developing countries are beset by more egregious manifestations of climate change. It is widely accepted that the detrimental effects of climate change will be visited largely on the countries least equipped to adapt, socially or economically.
Science says: 97% of climate experts agree humans are causing global warming.
Science achieves a consensus when scientists stop debating. When a question is first asked—like "what would happen if we put a load more CO2 in the atmosphere?"—there may be many hypotheses about cause and effect. Over a period of time, each idea is tested and retested (the scientific method) because reputation and kudos go to those who find the right answer. Nearly all hypotheses will fall by the wayside during this testing period, because only one is going to answer the question properly, without leaving all kinds of odd dangling bits that don't quite add up. Bad theories are usually rather untidy.
Eventually, the testing period must come to an end. The focus of investigation narrows down to those avenues that continue to make sense, that still add up, and quite often a good theory will reveal additional answers, or make powerful predictions that add substance to the theory. When Russian scientist Dmitri Mendeleev constructed the periodic table of elements, not only did he fit all known elements successfully, he predicted that elements we didn't even know about would turn up later on, and they did!
So a consensus in science is different from a political one. There is no vote. Scientists give up the debate because the sheer weight of consistent evidence is too compelling, the tide too strong to swim against any longer. Scientists change their minds on the basis of the evidence, and a consensus emerges over time. Not only do scientists stop debating, they also start relying on each others' work. All science depends on that which precedes it, and when one scientist builds on the work of another, he acknowledges the work of others through citations. The work that forms the foundation of climate change science is cited with great frequency by many other scientists, demonstrating that the theory is widely accepted and relied upon.
In the field of climate studies, which is informed by many different disciplines, the consensus is demonstrated by the number of scientists who have stopped debating what is causing climate change—and that's nearly all of them. A survey of all peer-reviewed papers on the subject "global climate change" published between 1993 and 2003 shows that not a single paper rejected the consensus position that global warming is man-made. 75% of the papers agreed with the consensus position while 25% made no comment either way, focusing on methods analysis instead (Oreskes 2004).
Several subsequent studies confirm that "...the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes." (Doran 2009). In other words, more than 95% of scientists working in the disciplines contributing to studies of our climate, accept that climate change is almost certainly being caused by human activities.
We should also consider official scientific institutions and what they think about climate change. There are no national or major scientific institutions anywhere in the world that dispute the theory of anthropogenic climate change. Not one.
In the field of climate science, the consensus is unequivocal: human activities are causing climate change.
Science says: All the indicators show that global warming is still happening.
When looking for evidence of global warming, there are many different indicators that we should look for. While it's natural to start with air temperature, a more thorough examination should be as inclusive as possible: snow cover, ice melt, air temperature over land and sea, even the sea temperature itself. A 2010 study included 10 key indicators, and as shown above, every one of them is moving in the direction expected of a warming globe.
The question of global warming stopping is often raised in the when weather events that we don't typically associate with global warming happen—a big winter storm or drought-relieving rain. Global warming is entirely compatible with these events; after all, they are just weather. In climate change, it is the long-term trends that are important; measured over decades or more, and the long term trends show that the globe is still warming.
Science says: Models successfully reproduce global temperature since 1900.
Climate models are mathematical representations of the interactions between the atmosphere, oceans, land surface, ice, and the sun. This is clearly a very complex task, so models are built to estimate trends rather than events. For example, a climate model can tell you it will be cold in winter, but it can't tell you what the temperature will be on a specific day—that's weather forecasting. Climate is weather averaged out over time, usually 30 years. Trends are important because they smooth out single events that may be extreme, but quite rare.
Climate models have to be tested to find out if they work. We can't wait for 30 years to see if a model is any good or not; models are tested against the past, against what we know happened. If a model can correctly predict trends from a starting point somewhere in the past, we could expect it to predict with reasonable certainty what might happen in the future.
So all models are first tested in a process called "hindcasting." The models used to predict future global warming can accurately map past climate changes. If they get the past right, there is no reason to think their predictions would be wrong. Testing models against the existing instrumental record suggested CO2 must cause global warming, because the models could not simulate what had already happened unless the extra CO2 was added to the model. All other known forcings are adequate in explaining temperature variations prior to the rise in temperature over the last thirty years, while none of them are capable of explaining the rise in the past thirty years. CO2 does explain that rise, and explains it completely without any need for additional, as yet unknown forcings.
Where models have been running for sufficient time, they have also been proved to make accurate predictions. For example, the eruption of Mt. Pinatubo allowed modelers to test the accuracy of models by feeding in the data about the eruption. The models successfully predicted the climatic response after the eruption. Models also correctly predicted other effects subsequently confirmed by observation, including greater warming in the Arctic and over land, greater warming at night, and stratospheric cooling.
The climate models, far from being alarmist, may be conservative in the predictions they produce. For example, here's a graph of sea level rise:
Sea level change; tide gauge data are indicated in red and satellite data in blue. The grey band shows the projections of the IPCC Third Assessment report (Copenhagen Diagnosis 2009).
Here, the models have understated the problem. In reality the events are all within the upper range of the model's predictions. There are other examples of models being too conservative, rather than alarmist as some portray them. All models have limits (uncertainties) for they are modeling chaotic systems. However, all models improve over time, and with increasing sources of real-world information such as satellites, the output of climate models can be constantly refined to increase their power and usefulness.
Climate models have already predicted many of the phenomena for which we now have empirical evidence. Climate models form a reliable guide to potential climate change.
Science says: The warming trend is the same in rural and urban areas, measured by thermometers and satellites.
Surveys of weather stations in the USA have indicated that some of them are not sited as well as they could be. This calls into question the quality of their readings.
However, when processing their data, the organizations which collect the readings take into account any local heating or cooling effects, such as might be caused by a weather station being located near buildings or near tarmacs at an airport. This is done, for instance, by weighting (adjusting) readings after comparing them against those from more rural weather stations nearby.
More importantly, for the purpose of establishing a temperature trend, the relative level of single readings is less important than whether the pattern of all readings from all stations taken together is increasing, decreasing, or staying the same from year to year. Furthermore, since this question was first raised, research has established that any error that can be attributed to poor siting of weather stations is not enough to produce a significant variation in the overall warming trend being observed. Even groups that have recreated the global temperature record on their own, with the intent to prove that there are problems with the data, have admitted that there is no substance to the claim.
It's also vital to realize that climate change not based simply on ground level temperature records. Other, completely independent temperature data compiled from weather balloons, satellite measurements, and from sea and ocean temperature records, also tell a remarkably similar warming story.
Confidence in climate science depends on the correlation of many sets of these data from many different sources in order to produce conclusive evidence of a global trend.
Science says: Global change is happening too fast for the ecosystem to adapt.
Humans are transforming the global environment. Great swathes of temperate forest in Europe, Asia, and North America have been cleared over the past few centuries for agriculture, timber, and urban development. Tropical forests are now on the front line. Human-assisted species invasions of pests, competitors, and predators are rising exponentially, and over-exploitation of fisheries and forest animals for bush meat continues to be the rule rather than the exception.
Given these existing pressures and upheavals, it is a reasonable question to ask whether global warming will make any further meaningful contribution to this mess. Some see no danger at all, maintaining that a warmer planet will be beneficial for mankind and other species on the planet, and that "corals, trees, birds, mammals, and butterflies are adapting well to the routine reality of changing climate."
However, global warming has already affected the ecosystem by changing where some species can survive, the timing of breeding, migration, flowering, and so on. Although it's difficult to predict the future extinction risk, some scientists have found that 18 to 35% of plant and animal species will be committed to extinction by 2050 due to climate change.
A large number of ancient mass extinction events have indeed been strongly linked to global climate change, including the most sweeping die-off that ended the Palaeozoic Era, 250 million years ago, and the less cataclysmic Palaeocene-Eocene Thermal Maximum, 55 million years ago. These extinctions were natural, and there are a number of reasons why today's global change will have a particularly severe impact on biodiversity:
Science says: 2010 is globally the hottest year on record, tied with 2005.
No, it hasn't been cooling since 1998. Even if we ignore long term trends and just look at the record-breakers, that wasn't the hottest year ever. Different reports show that, overall, 2005 was hotter than 1998. What's more, globally, the hottest 12-month period ever recorded was from June 2009 to May 2010.
Though humans love record-breakers, they don't, on their own, tell us a much about trends, and it's trends that matter when monitoring Climate Change. Trends only appear by looking at all the data, globally, and taking into account other variables—like the effects of the El Nino ocean current or sunspot activity—not by cherry-picking single points.
There's also a tendency for some people just to concentrate on air temperatures when there are other, more useful, indicators that can perhaps give us a better idea how rapidly the world is warming. Oceans for instance, due to their immense size and heat storing capability, tend to give a much more 'steady' indication of the warming that is happening. Here records show that the Earth has been warming at a steady rate before and since 1998 and there's no signs of it slowing any time soon.
Science says: Satellites measure Antarctica is gaining sea ice but losing land ice at an accelerating rate, which has implications for sea level rise.
Skeptic arguments that Antarctica is gaining ice frequently hinge on an error of omission, namely ignoring the difference between land ice and sea ice.
In glaciology, and particularly with respect to Antarctic ice, not all things are created equal. Let us consider the following differences. Antarctic land ice is the ice which has accumulated over thousands of years on the Antarctica landmass itself through snowfall. This land ice therefore is actually stored ocean water that once fell as precipitation. Sea ice in Antarctica is quite different as it is generally considered to be ice which forms in salt water primarily during the winter months.
Estimates of Total Antarctic Land Ice Changes and approximate sea level contributions using many different measurement techniques. Adapted from The Copenhagen Diagnosis. (CH= Chen et al. 2006, WH= Wingham et al. 2006, R= Rignot et al. 2008b, CZ= Cazenave et al. 2009 and V=Velicogna 2009)
In Antarctica, sea ice grows quite extensively during winter but nearly completely melts away during the summer (see above). That is where the important difference between antarctic and arctic sea ice exists. Arctic sea ice lasts all the year round, there are increases during the winter months and decreases during the summer months but an ice cover does in fact remain in the North which includes quite a bit of ice from previous years (Figure 1). Essentially Arctic sea ice is more important for the earth's energy balance because when it melts, more sunlight is absorbed by the oceans whereas Antarctic sea ice normally melts each summer leaving the earth's energy balance largely unchanged.
One must also be careful how you interpret trends in Antarctic sea ice. Currently this ice is increasing and has been for years but is this the smoking gun against climate change? Not quite. Antarctic sea ice is gaining because of many different reasons but the most accepted recent explanations are listed below:
All the sea ice talk aside, it is quite clear that really when it comes to Antarctic ice, sea ice is not the most important thing to measure. In Antarctica, the most important ice mass is the land ice sitting on the West Antarctic Ice Sheet and the East Antarctic Ice Sheet.
Estimates of recent changes in Antarctic land ice (see above) range from losing 100 gigatons/year to over 300 gigatons/year. Because 360 gigatons/year represents an annual sea level rise of 1 mm/year, recent estimates indicate a contribution of between 0.27 mm/year and 0.83 mm/year coming from Antarctica. There is of course uncertainty in the estimations methods but multiple different types of measurement techniques (explained here) all show the same thing, Antarctica is losing land ice as a whole, and these losses are accelerating quickly.
Skeptical Science was founded by physicist John Cook in 2007 to explore what science has to say about global warming. In 2011, Skeptical Science won the Australian Museum Eureka Prize for the Advancement of Climate Change Knowledge. It is not affiliated with any organization, and is funded by contributions from readers.
John Cook is the Climate Change Communication Fellow for the Global Change Institute at the University of Queensland. He created and runs skepticalscience.com. His efforts have concentrated on making climate science accessible to the general public, releasing smartphone apps for the iPhone and Android phones. He has produced climate communication resources adopted by organizations such as NOAA and the U.S. Navy, and co-authored the book Climate Change Denial: Heads in the Sand with environmental scientist Haydn Washington.