The Most Common Skeptical Arguments About Climate Change (from the Grist articles: How to Talk to a Climate Skeptic – see site for images)

There is no evidence

Objection: Despite what the computer models tell us, there is actually no evidence of significant global warming.

Answer: Global warming is not an output of computer models; it is a conclusion based on observations of a great many global indicators. By far the most straightforward evidence is the actual surface temperature record. While there are places — in England, for example — that have records going back several centuries, the two major global temperature analyses can only go back around 150 years due to their requirements for both quantity and distribution of temperature recording stations.

These are the two most reputable globally and seasonally averaged temperature trend analyses:

NASA GISS direct surface temperature analysis
CRU direct surface temperature analysis

Both trends are definitely and significantly up. In addition to direct measurements of surface temperature, there are many other measurements and indicators that support the general direction and magnitude of the change the earth is currently undergoing. The following diverse empirical observations lead to the same unequivocal conclusion that the earth is warming:

Satellite Data
Borehole analysis
Glacial melt observations
Sea ice melt
Sea level rise
Proxy Reconstructions
Permafrost melt

There is simply no room for doubt: the Earth is undergoing a rapid and large warming trend.

The temperature record is simply unreliable

Objection: The surface temperature record is full of assumptions, corrections, differing equipment and station settings, changing technology, varying altitudes, and more. It is not possible to claim we know what the “global average temperature” is, much less determine any trend. The IPCC graphs only say what the scientists want them to say.

Answer: There is actually some truth to the part about the difficulties; scientists have overcome many of them in turning the hundreds of thousands of measurements taken in many different ways and over a span of more than a dozen decades into a single globally averaged trend.

But this is the nature of science — no one said it was easy. It’s taken the scientific community a long time to finally come out and say that what we have been observing for 100 years is in fact exactly what it looks like. All other possible explanations (for example, the Urban Heat Island effect) have been investigated, the data has been examined and re-examined, reviewed and re-reviewed, and the conclusion has become unassailable.

And while it is true that differing weather station locations, from proximity to lakes or rivers or elevation above sea level, probably make it impossible to arrive at a meaningful figure for global average surface temperature, that is not what we are really interested in. The investigation is focused on trends, not the absolute level. Often, as in this case, it is easier to determine how much a given property is changing than what its exact value is. If one station is near an airport at three feet above sea level and another is in a park at 3000 feet, it doesn’t really matter — they both show rising temperature, and that is the critical information.

So how do we finally know when all the reasoning is reasonable and the corrections correct? One good way is to cross check your conclusion against other completely unrelated data sets. In this case, all the other available indicators of global temperature trends unanimously agree. Go ahead, put aside the direct surface temperature measurements — global warming is also indicated by:

Satellite measurements of the upper and lower troposphere
Weather balloons show very similar warming
Borehole analysis
Glacial melt observations
Declining arctic sea ice
Sea level rise
Proxy Reconstructions
Rising ocean temperature

All of these completely independent analyses of widely varied aspects of the climate system lead to the same conclusion: the Earth is undergoing a rapid and substantial warming trend. Looks like the folks at NASA and CRU know what they are doing after all.

One hundred years is not enough

Objection: One hundred and some years of global surface temperatures is not long enough to draw any conclusions from or worry about anyway.

Answer: The reliable instrumental record only goes back 150 years in the CRU analysis, 125 in the NASA analysis. This is a simple fact that we are stuck with. 2005 was the warmest year recorded in that period according to NASA, a very close second according to CRU. Because of this limit, it is not enough to say today that these are the warmest years since 150 years ago, rather one should say ‘at least’:

1998 and 2005 are the warmest two years in at least the last 150.

But there is another direct measurement record available that can tell us things about temperature over the last 500 years, and that is borehole measurements. This involves drilling a deep hole and measuring the temperature of the earth at various depths. It gives us information about century-scale temperature trends, as warmer or cooler pulses from long term surface changes propagate down through the crust.

Using this method we can see that temperatures have not been consistently this high as far back as this method allows us to look. This way of inferring surface temperatures does smooth out yearly fluctuations and even short term trends, so we can not know anything directly about individual years. But given the observable range of inter-annual variations recorded over the last century, it is quite reasonable to rule out single years or even decades being far enough above the baseline to rival today.

Using this record, we can reasonably conclude that it is warmer now than any time in at least the last 500 years.

It is possible to make reconstructions of temperature much further back, using what are called proxy data. These include things like tree rings, ocean sediment, coral growth, layers in stalagmites, and others. The reconstructions available are all slightly different and provide sometimes more and sometimes less global versus regional coverage over the last one or two thousand years. Note: this covers what is often referred to as the Medieval Warm Period. As noted, all these reconstructions are different, but they all show some similar patterns of temperature change over the last several centuries. Most striking is the fact that each record reveals that the 20th century is the warmest of the entire record, and that warming was most dramatic after 1920.

Thus, we can reasonably say it is warmer now than any other time in at least the last 1,000 years.

The only other candidate for a higher temperature period — going back through the entire Holocene (~10,000bp to now) — is called the Holocene Climatic Optimum some 6,000 years ago. It is not known exactly what the temperatures were then; the farther back in time we try to look, the greater the uncertainties. Even so, the Holocene Climatic Optimum has long been cautiously thought to be almost as warm or even warmer than now.

That conclusion is starting to look less likely, as it has been determined that the anomalous warmth of that time was actually confined to the northern hemisphere and occurred only in the summer months.

Robert Rohde’s website, Global Warming Art, has a nice graph of many reconstructions of Holocene temperature, regional and global, all super-imposed with an average of all of them combined, shown below. This represents the best estimate available of global temperatures in the Holocene.

Thus, we can reasonably believe it is warmer now than at any other time in at least the last 10,000 years.

Before the current interglacial, the planet was in the grip of a much colder glacial period with ice sheets well down into the continental U.S. This period ended just some 11,000 years ago. The record of glacial-interglacial cycles can be read in Antarctic ice core analysis, and it shows these cycles over many 100Kyr periods. The IPCC offers a good version of this graph.

If our reading of the Holocene is correct, it is warmer now than at any other time in over the last 100,000 years.

And that is a bit more than 100 years. It is, in fact, the entire history of our species.

Current global warming is just part of a natural cycle

Objection: Current warming is just part of a natural cycle.

Answer: While it is undoubtedly true that there are natural cycles and variations in global climate, those who insist that current warming is purely natural — or even mostly natural — have two challenges.

First, they need to identify the mechanism behind this alleged natural cycle. Absent a forcing of some sort, there will be no change in global energy balance. The balance is changing, so natural or otherwise, we need to find this mysterious cause.

Second, they need to come up with an explanation for why a 35% increase in the second most important greenhouse gas does not affect the global temperature. Theory predicts temperature will rise given an enhanced greenhouse effect, so how or why is it not happening?

The mainstream climate science community has provided a well-developed, internally consistent theory that accounts for the effects we are now observing. It provides explanations and makes predictions. Where is the skeptic community’s model or theory whereby CO2 does not affect the temperature? Where is the evidence of some other natural forcing, like the Milankovich cycles that controlled the ice ages (a fine historical example of a dramatic and regular climate cycle that can be read in the ice core records taken both in Greenland and in the Antarctic)?

Is this graph a candidate for explaining today’s warming? A naive reading of this cycle indicates we should be experiencing a cooling trend now — and indeed we were gradually cooling over the length of the pre-industrial Holocene, around .5C averaged over 8,000 years.

Not only is the direction of the change wrong, but compare the speed of those fluctuations to today’s changes. Leaving aside the descents into glaciation, which were much more gradual, the sudden (geologically speaking) jumps up in temperature every ~100,000 years represent a rate of change roughly ten times slower what we are currently witnessing.

So could current changes be part of a natural cycle? Well, no natural cause has been identified. There is no climatological theory in which CO2 does not drive temperature. And natural cycle precedents do not exhibit the same extreme changes we’re now witnessing.

In short: No.

It’s cold today in Wagga Wagga

Objection: It was way colder than normal today in Wagga Wagga, proof that there is no global warming.

Does this even deserve an answer? If we must …

Answer: The chaotic nature of weather means that no conclusion about climate can ever be drawn from a single data point, hot or cold. The temperature of one place at one time is just weather, and says nothing about climate, much less climate change, much less global climate change.

What’s Wrong with warmer weather?

Objection: The earth has had much warmer climates in the past. What’s so special about the current climate? Anyway, it seems like a generally warmer world will be better.

Answer: I don’t know if there is a meaningful way to define an “optimum” average temperature for planet earth. Surely it is better now for all of us than it was 20,000 years ago when so much land was trapped beneath ice sheets. Perhaps any point between the recent climate and the extreme one we may be heading for, with tropical forests inside the arctic circle, is as good as any other. Maybe it’s even better with no ice caps anywhere.

It doesn’t matter. The critical issue is not what the temperature is, or may be, or will be. The critical issue is how fast it is moving.

Rapid change is the real danger. Human habits and infrastructure are suited to particular weather patterns and sea levels, as are ecosystems and animal behaviors. The rate at which global temperature is rising today is likely unique in the history of our species.

This kind of sudden change is rare even in geological history, though perhaps not unprecedented. So the planet may have been through similar things before — that sounds reassuring, right?

Not so much. Once you look at the impact similar changes had on biodiversity at the time, the existence of historical precedent becomes anything but reassuring. Rapid climate change is the prime suspect in most mass extinction events, including the Great Dying some 250 million years ago, in which 90% of all life went extinct.

What we know about ecosystems, and what geologic history demonstrates, is that dramatic climate changes — up or down or sideways — are a tremendous shock to the biosphere and cause mass extinction events. That, all in all, is not likely to be a good thing.

Warming is due to the Urban Heat Island effect

Objection: The apparent rise of global average temperatures is actually an illusion due to the urbanization of land around weather stations, the Urban Heat Island effect.

Answer: Urban Heat Island Effect has been examined quite thoroughly (PDF) and found to have a negligible effect on temperature trends. Real Climate has a detailed discussion of this here. What’s more, NASA GISS takes explicit steps in their analysis to remove any such spurious signal by normalizing urban station data trends to the surrounding rural stations. It is a real phenomenon, but it is one climate scientists are well aware of and have taken any required steps to remove its influence from the raw data.

But heavy duty data analysis and statistical processing aside, a little common sense and a couple of pertinent images should put this idea to bed. Here is an image, taken from Astronomy Picture of the Day (a wonderful site, by the way), of the surface of the earth. It is a composite of hundreds of satellite images all taken at night. (The large version is well worth the download time!)

Aside from being very beautiful, it is a perfect indicator of urbanization on earth. As you can see, the greatest urbanization is over the continental United States, Europe, India, Japan, Eastern China, and generally coastal South America.

This next image was taken from NASA GISS. It is a global surface temperature anomaly map which shows warming (and infrequently, cooling) by region.

Look at North America, look at Europe, at Asia, Australia, Africa and the Poles and compare them to the urbanization in the image from APOD. There is quite simply no way to discern any correlation whatsoever between urbanization and warming. If the UHI effect were the cause of warming in the globally averaged record, we would see it in this map.

The claim that global warming is an artifact of the Urban Heat Island Effect is simply an artifact of the Urban Myth Effect.

Addendum: Wikipedia has a very good article on this subject. Among all the interesting details it mentions a few papers that directly discuss efforts to identify and quantify UHI influences on the global temperature trend including this one (PDF), which would be a good one to cite:

A 2003 paper (“Assessment of urban versus rural in situ surface temperatures in the contiguous United States: No difference found”; J climate; Peterson; 2003) indicates that the effects of the urban heat island may have been overstated, finding that “Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures.” This was done by using satellite-based night-light detection of urban areas, and more thorough homogenisation of the time series (with corrections, for example, for the tendency of surrounding rural stations to be slightly higher, and thus cooler, than urban areas). As the paper says, if its conclusion is accepted, then it is necessary to “unravel the mystery of how a global temperature time series created partly from urban in situ stations could show no contamination from urban warming.” The main conclusion is that micro- and local-scale impacts dominate the meso-scale impact of the urban heat island: many sections of towns may be warmer than rural sites, but meteorological observations are likely to be made in park “cool islands.”

If necessary, be sure to refer to all the other ways we know the global warming trend is not an artifact of anything. It is real.

The satellites show cooling

Objection: Satellite readings, which are much more accurate, show that the earth is in fact cooling.

I wonder how long before this one stops coming up?

Answer: There are a few advantages to the satellite readings,mainly the more uniform global coverage and the fact that readings can be taken at different altitudes. However, it is an extremely complicated process which uses microwaves emitted by the oxygen in the atmosphere as a proxy for temperature.

The complications arise from many things, including decay of the satellite orbits, splicing together and calibrating records from different instruments, trying to separate the signals by the layer of atmosphere they originate from, etc. It is a little ironic that the same people who distrust the surface record so happily embrace this even-more-convoluted exercise in data processing!

Anyway, it has been many years since the satellite analysis showed cooling.

Until recently, though, one of the many analyses of tropospheric temperatures did show very little warming and was in direct contradiction to model predictions that say the troposphere should warm significantly in an enhanced greenhouse environment. Something had to be wrong, the observations or the model predictions. Naturally, the skeptics had no doubt it was the models that were off.

However, it turns out that additional errors were uncovered and the MSU Satellite temperature analysis now shows warming well in line with model expectations. Real Climate has a good rundown of the technical details for those with the stomach for it. In short, this long-running debate turned out to be a great validation of the models and a real death blow to the “earth is not warming” crowd.

Beware of zombies!

Global warming stopped in 1998

Objection: Global temperatures have been trending down since 1998. Global warming is over.

Answer: At the time, 1998 was a record high year in both the CRU and the NASA GISS analyses. In fact, it blew away the previous record by .2 degrees C. (That previous record went all the way back to 1997, by the way!)

According to NASA, it was elevated far above the trend line because 1998 was the year of the strongest El Nino of the century. Choosing that year as a starting point is a classic cherry pick and demonstrates why it is necessary to remove chaotic year-to year-variability (aka: weather) by smoothing out the data. Looking at CRU’s graph below, you can see the result of that smoothing in black.

Clearly 1998 is an anomaly and the trend has not reversed. (Even the apparent leveling at the end is not the real smoothing. The smoothed trend in 2005 depends on all of its surrounding years, including a few years still in the future.) By the way, choosing the CRU analysis is also a cherry pick — NASA has 2005 breaking the 1998 record, though by very little.

Now, this is an excusable mistake for average folks who do not need the rigors of statistical analysis in their day jobs. But any scientist in pretty much any field knows that you cannot extract meaningful information about trends in noisy data from single-year end points. It’s hard to hear a scientist make this argument and still believe they speak with integrity in this debate — seems more like an abuse of the trust placed in them as scientists. Bob Carter is just such a voice, and was the first to trot out this argument in an article in the Daily Telegraph. Since then it has echoed far and wide and been used by Richard Lindzen as well as a host of skeptic websites.

Interestingly, Bob Carter seems to know what he is doing. He tries to pre-empt objections in his article by insinuating that any choice of starting point (say, 1978) will just be a cherry pick with the opposite motive! But cherry picking is about choosing data for the sole purpose of supporting a pre-conceived conclusion. It is not the simple act of choosing at all. One must choose some starting point. In the case of his example year, 1978, it’s often chosen simply because it is the first year that satellite records of tropospheric temperatures were available.

So what choices are there? What are the reasons for those choices? What conclusions we can draw from them?

As mentioned above, you could choose to examine the last 30 years — that is when both surface and tropospheric readings have been available. We have experienced warming of approximately .2 degrees C/decade during this time. It would take a couple of decades trending down before we could say the recent warming ended in 1998.
You could choose 1970 in the NASA GISS analysis — the start of the late 20th century warming, and as such a significant feature of the temperature record. The surface temperature over this period shows .6 degrees C warming.
You could choose 1965 in the CRU analysis — when the recent warming started in their record. It shows around .5 degrees C warming of the smoothed trend line.
You could choose 1880 in the NASA record — it shows .8 degrees C warming.
You could choose 1855 in the CRU record — it shows .8 degrees C warming. As with the trend above, we can not say it is over without many decades more data indicating cooling.
You could choose to look at the last 500 years in the bore hole record analysis — that is its entire length. It puts today about 1 degree C above the first three centuries of that record. In that kind of analysis, today’s record will be hidden from view for many decades.
You could choose to look at the last 1,000 years, because that is as far back as the dendrochronology studies reliably go. Then the conclusion is:

Although each of the temperature reconstructions are different (due to differing calibration methods and data used), they all show some similar patterns of temperature change over the last several centuries. Most striking is the fact that each record reveals that the 20th century is the warmest of the entire record, and that warming was most dramatic after 1920.

You could choose to look at the entire period of time since the end of the last ice age, around 10,000 years ago. Then the conclusion is that GHG warming has reversed a long and stable period of slight downward trend, and we are now at a global temperature not experienced in the history of human civilization — the entire Holocene. It will be many centuries until such a long view of today’s climate is available. The situation is a bit more urgent than that!

That about covers any period of time relevant to today’s society. “It has stopped warming” is only supported by selecting a single year out of context and using a seven-year window to look at multi-decadal trends in climate. That’s a classic cherry pick.

They predicted global cooling in the 1970s

Objection: The alarmists were predicting the onset of an ice age in the ’70s. Now it’s too much warming! Why should we believe them this time?

Answer: It is true that there were some predictions of an “imminent ice age” in the 1970s, but a cursory comparison of those warnings and today’s reveals a huge difference.

Today, you have a widespread scientific consensus, supported by national academies and all the major scientific institutions, solidly behind the warning that the temperature is rising, anthropogenic CO2 is the primary cause, and it will worsen unless we reduce emissions.

In the 1970s, there was a book in the popular press, a few articles in popular magazines, and a small amount of scientific speculation based on the recently discovered glacial cycles and the recent slight cooling trend from air pollution blocking the sunlight. There were no daily headlines. There was no avalanche of scientific articles. There were no United Nations treaties or commissions. No G8 summits on the dangers and possible solutions. No institutional pronouncements. You could find broader “consensus” on a coming alien invasion.

Quite simply, there is no comparison.

If you want some additional detail, Real Climate has discussed this, and William Connelly has made a hobby of gathering everything that was written about global cooling at the time.

Some sites show cooling

Objection: Some stations, in the U.S. for example, show cooling trends. If there were really global warming, it would be warming everywhere.

Answer: Global warming is the long-term increase in globally and seasonally averaged surface temperatures. It is not the case, nor is it expected, that all regions on the planet, let alone all weather stations, will show the same changes in temperature or rainfall patterns. Many stations have shown cooling, and some small regions have shown modest cooling as well. This does not invalidate global warming theory; it is merely the result of regional variation, and an example of how varied and complex the climate system is.

The contrarian website CO2 Science makes this fallacious argument part of its homepage by featuring a “Station of the Week” that’s exhibited trends significantly different from the global one. Given the effort and technical content behind that website, and the fact that they prominently display this intellectual sleight of hand, I think it is safe to say they are simply being dishonest.

All of the various global temperature trend analyses show significant warming in the average temperature:

Hadley Centre

These analyses agree with the expectations of climate theory, as well as all the other lines of evidence.

One record year is not global warming

Objection: So 2005 was a record year. Records are set all the time. One really warm year is not global warming.

Answer: This is actually not an unreasonable point — single years taken by themselves can not establish or refute a trend. So 2005 being the hottest globally averaged temperature on record is not convincing. Then how about:

every year since 1992 has been warmer than 1992;
the ten hottest years on record occurred in the last 15;
every year since 1976 has been warmer than 1976;
the 20 hottest years on record occurred in the last 25;
every year since 1956 has been warmer than 1956; and
every year since 1917 has been warmer than 1917.

The five-year mean global temperature in 1910 was .8 degrees Celsius lower than the five year mean in 2002. This, and all of the above, comes from the temperature analysis by NASA GISS.

There is an interesting quote from that page:

Record warmth in 2005 is notable, because global temperature has not received any boost from a tropical El Niño this year. The prior record year, 1998, on the contrary, was lifted 0.2°C above the trend line by the strongest El Niño of the past century.

So, yes it is true that one record year does not make a long term trend, but that is clearly not the whole story.