Responses to popular denialist arguments:

See also

Denialist Myths FAQ(more details)

"It's not happening, it's not us causing it, but it's really quite a good thing that we are" - how The Oz covers all bases.

"Global warming has plateaued"

Many sceptics point to the years of temperature records since 1998 and claim they show global warming has ceased.  Certainly there is a pause, but the graph for the last 100 years shows several such pauses, while the long term trend is clear.

Has it really slowed?

It is impossible to get a perfect snapshot of the Earth's average temperature at one instant. Satellites don't show everything. Some of the heat may have gone where we have few sensors, such as the deep ocean or under icecaps. Neither can they distinguish ice at 0C from water at 0C, but the latter represents a lot more heat: just as melting ice in the Esky keeps the drinks cold for a while, melting ice at the poles temporarily cools the oceans.

 Satellite data do show how much heat is coming in from the sun and that less is radiating back out.  The difference is being stored somewhere, but current measurements only find about half.

For such reasons, the graphs are pretty erratic over periods of ten or 15 years.  You have to look at periods of 25 or 30 years before a consistent picture emerges.

See also

If it really has slowed, why?

In January 2010, NOAA reported that relative humidity in a critical part of the atmosphere had suddenly and unaccountably dropped.  This could well explain a pause in the rise of temperatures.  The bad news is that would mean the pause is only temporary, and, worse, could bounce back to make up for the pause.

Another possibility is sulphate particles.  It has been discovered (Science, DOI:10.1126/science.1182274) that the monsoon can thrust such pollution in the stratosphere, where it can last much longer as a sunshade.

 Feb 2014: Excess heat found deep in Pacific

Sea level as an indicator

A more reliable test of the short-term trend is to look at global sea level.  The rise comes from expansion of the water as it warms, at any depth, plus any extra water from melted ice.  Either way, it indicates warming.  Although it only has such data for the last 17 years, this graph shows a very clear picture:

On the other hand, there is a lag in sea level changes, so it does not give quite the latest picture.

Sep 2011: Models suggest extra heat has gone into ocean deeps

Nov 2013: Warming ocean depths theory gains support


Extracts from hacked emails from the University of East Anglia's Climate Research Unit (CRU), popularly known as "Climategate", have provided ammunition for the sceptics. On closer inspection, however, it may be their only fault was to be less open than they might have been.

Some aspects of the furore are certainly a beat-up. The reference to using a certain "trick" with the data is normal techie-speak for a neat method, in this case one for extracting answers from raw data. There's no implication in this instance that the method is not correct.

The purported suppression of converse findings consisted of avoiding lending unwarranted credibility to non-peer-reviewed papers by their juxtaposition with peer-reviewed papers in the same journal.

For a good analysis see

April 2010 update: a second inquiry has cleared the researchers of any wrongdoing, falsification, or bias.  It has, however, found fault with their statistical methods and recommends working more closely with professional statisticians.

July 2010 update: a third inquiry, by Penn State University, has cleared Michael Mann of manipulating or falsifying data.

"CO2 increase follows temperature increase"

In the geological record that is true, and well understood.
The change in CO2 was not the trigger in those cases. Rather, some other known event triggered the initial increase, such as reduced volcanism or increased sun activity.  When those other causes declined, the oceans cooled a little and reabsorbed some CO2, leading to further cooling, etc.

But those triggers alone do not explain the eventual temperature rise that occurred. The initial rise caused an increase in CO2 (for well understood reasons, such as CO2 release from soils and seas) which then magnified it. This feedback resulted in the eventual rise being far more than it would otherwise have been, and is one of the reasons what we are doing now is so scary. Worse, we are adding CO2 into this system which was not then available for release.

(You might wonder why this does not go on forever, leading to more and more GHGs and higher and higher temperatures. The feedback is not a simple straight-line relationship. Eventually, extra GHG has less effect. At the same time, the hotter Earth radiates more heat into space, leading to a new equilibrium.)

However, the fact that the initial trigger was a temperature change on those occasions does not explain the extent of the lag.

The recorded temperatures and CO2 are both at the Antarctic. While the CO2 can be expected to represent the global state, the temperatures may be more local. Andrey Ganopolski and D.M. Roche show that temperature changes tend to be greatest in the Antarctic first, whereas CO2 levels are driven more by temperatures elsewhere. Hence the data show temperature changes preceding CO2 changes.

Apr 2012: CO2 rise caused end of last ice age

"The expected 'hotspot' in the troposphere is missing"

This is a very technical argument, but it has been adequately debunked.

In short, the "hotspot" is not a predicted fingerprint of warming due to increased GHGs. It is predicted to exist regardless of any warming or any cause of such warming. So its absence did suggest the models are not perfect, but it did not point suspicion away from GHGs.

Recently it has been shown that the measurements were unreliable, and the hotspot may have been found. See also

Meanwhile, there is a fingerprint predicted for GHG-induced warming: as the blanket of GHGs holds in more heat in the lower levels of the atmosphere, the stratosphere above cools and shrinks (leading to the darkly humorous remark that the sky really is falling!) That would not be expected if, for example, the cause were extra output from the sun. Yet cooling in the stratosphere is indeed what we see.

Nov 2010: Hotspot found - official!

June 2015: Hotspot even stronger than predicted, no slowdown

"Non-believers don't have to prove anything".

Wrong. This is not a criminal court case, where CO2 is innocent until proven guilty. It's a matter of risk management. The financial cost of making serious strides to reduce emissions is as nothing to that of a 3oC rise in global temperature.

In the last 100 years we have increased the CO2 level way beyond what it had ever been during mankind's previous existence. We are in unknown climatic territory. As with the unleashing of a new drug or food product on the public, the burden of proof is with those doing so.

Australia's emissions are small (1.5%)

It is quite true that if Australia is the only country to act it will hardly help at all.  But those countries have huge populations. Would it make sense to say that each country should be allowed the same emissions, regardless of population? Of course not!

It is the silliest argument of all.  It's on a par with "my taxes are less than a millionth of the total, so it doesn't matter if I don't pay them".  Why should an individual be entitled to more emissions just because he/she lives in a country with a small population?

Ultimately, every country needs to become carbon neutral anyway (or engage in some financial arrangement by which another country draws down what that country emits).

The countries which develop and adopt the required technology soonest will then be the best placed economically. Meanwhile, does it make sense for Australia to take a lead? Yes and no. There will short term economic cost in going it alone, but this has to be balanced against the overwhelming need to encourage the others. If each country waits for the others we all lose.

"The CO2 greenhouse effect is already saturated"

Some denialists argue that the CO2 in the atmosphere is already having as much greenhouse effect as it can, so adding more won't hurt. That's sort of true if you only look at a fixed height layer of air close to ground level. But what matters is the thickness of the blanket.

The effective thickness now is about 5km. From that height, where the temperature is -18oC, it radiates heat into space at the same rate that the sun pours energy in. As you go down through the atmosphere, the air gets warmer at 6.5oC per km. So down here the average is about +14.5oC. More CO2 thickens the blanket, raising the altitude of this -18oC point and increasing the temperature at ground level.

The effect on Venus is dramatic: its atmosphere is 96% CO2 and the temperature at the surface is 467oC, hotter even than Mercury. With no greenhouse effect, it has been estimated the temperature there would be below -10oC.



In 2007 the IPCC reported that glaciers in the Himalayas could vanish in 30 years. The British Sunday Times claimed that was based on a conversation between a New Scientist journalist and a single Indian scientist a decade ago and was never published as peer-reviewed research. An alternative explanation is that it was a miscopying of the date 2350 as 2035. Either way, the consensus is now that:

  • The Himalayan glaciers are not, on average, retreating faster than glaciers generally.
  • Some are advancing.
  • Changes in glacier length are more to do with precipitation and winds than temperature, and GW will affect these variously.

How does this affect the GW debate? It ought not affect it much:

  • There is a huge delay between what we do and the effect on the climate. We have to act now to avoid consequences in 300 years.
  • Rising temperatures will alter wind and weather in ways we are still unable to predict accurately, so a risk of suddenly retreating glaciers - anywhere - remains.
  • Glacier retreat is only one of many dangers from GW.

But of course, it is fodder for the denialists, so expect an avalanche of criticism for the IPCC.



Arctic Sea Ice is stable

Arctic sea ice extent reached a historical minimum in October 2007, prompting claims that it could be ice-free in summer in 10 or 20 years. In 2008 and 2009, summer extent recovered somewhat.

Two points must be borne in mind:

  • What matters is sea ice volume. The ice has thinned, so the decline continues.
  • The main cause of the melt has been patterns of wind and ocean currents, transporting more heat into the Arctic. Of course, such changes may themselves have resulted from global warming, but it is too early to say.

Whatever the explanation, sea ice melt will reduce albedo, resulting in more warming. Total melt would threaten the Greenland ice sheet, and if that goes sea levels would rise by metres (but this will take centuries).


May 2010:,8599,1956932,00.html


Upper Troposphere is drying

The models used to predict the degree of warming assume that relative humidity in the atmosphere (most importantly, the upper troposphere) stays roughly constant. So as the GHGs we add raise the temperature, more water vapour is added. Since water vapour is a strong GHG, this amplifies the effect.

Some measurements had cast doubt on this, indicating that the relative humidity is dropping:

Theor Appl Climatol, DOI 10.1007/s00704-009-0117-x, Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data Garth Paltridge & Albert Arking & Michael Pook; Received: 21 July 2008 / Accepted: 4 February 2009

The data come from radiosondes, which are sent up on weather balloons. The authors themselves remark "It is accepted that radiosonde-derived humidity data must be treated with great caution". It certainly appeared contradicted by satellite data - see below. Furthermore, it uses "reanalysis". This employs data from different sources at different periods of time. That introduces time-based biases and can appear to show trends which are not there in the individual datasets. (See also Sherwood, S. C., R. Roca, T. M. Weckwerth and N. G. Andronova, Tropospheric water vapor, convection and climate. Reviews of Geophysics, In Press as at Jan 2010.)

In January 2010 NOAA researchers reported there was a real and sudden drop in relative humidity of the stratosphere by about 10% around the year 2000.  However, stratospheric RH is less important, and the drop was a one-off.

Satellite data contradicting a drop in upper-tropospheric relative humidity:

Dessler, A. E., Z. Zhang, and P. Yang (2008), Water-vapor climate feedback inferred from climate fluctuations, 2003-2008 Geophys. Res. Lett., 35, L20704

Gettelman A and Fu, Q. (2008) Observed and simulated upper-tropospheric water vapor feedback J. Climate 21, 3282-3289

Buehler SA (2008) An upper tropospheric humidity data set from operational satellite microwave data J. Geophys. Res. 113, art #D14110

Brogniez H and Pierrehumbert RT (2007) Intercomparison of tropical tropospheric humidity in GCMs with AMSU-B water vapor data. Geophys. Res. Lett. 34, art #L17912

Santer BD et al. (2007) Identification of human-induced changes in atmospheric moisture content. Proc. Natl. Acad. Sci. USA 104, 15248-15253

Soden BJ, et al (2005) The radiative signature of upper tropospheric moistening Science 310, 841-844.

That last includes: "Although an international network of weather balloons has carried water vapor sensors for more than half a century, changes in instrumentation and poor calibration make such sensors unsuitable for detecting trends in upper tropospheric water vapor."

CO2 Rise Linear

Some denialists claim that the IPCC is assuming CO2 levels in the atmosphere will rise exponentially, whereas observations show a straight line.

  • Any smooth-ish curve will look like a straight line if you zoom in on a sufficiently short part of it. A ship may appear to be steaming in a dead straight line, but it has to follow the curve of the Earth's surface. The curve is clear over 50 years or more.
  • The data support the view that the level of CO2 is accelerating. In the sixties it was growing at under 1ppmv annually; now it's at 2 to 3ppmv annually.
  • The IPCC has not assumed atmospheric concentrations will increase exponentially. They do predict acceleration (faster-than-linear growth), but the "exponential" tag comes from the denialists' interpretation.
  • Our emissions change according to the economic cycle and technology, so such predictions need to be based on forecasts of those. Long term, economies have grown exponentially. Although it's physically impossible for that to go on forever, it could continue for some decades yet if we don't wake up to the need for zero growth. Exponentially growing emissions do not imply exponentially increasing concentrations in the atmosphere, but they do suggest something faster than linear.
  • It is also important to predict how fast the oceans will absorb extra CO2, and this certainly appears to be slowing. As they warm, it is expected to go into reverse.

It has even been claimed that the CO2 concentration has not risen in 160 years! This seems to be from a misunderstanding of a finding that the fraction of emissions that stays in the atmosphere has not changed in that time.

Medieval Warm Period

Graphs have been published purporting to show that the MWP was warmer than today. Much is made of the presence then of a wine industry in England.

The MWP was a patchy phenomenon, occurring mostly in the northern hemisphere, and in different places at different times. See also It's the Sun.  Taking the whole Earth's temperature, from the best available evidence, removes much of the blip.

Often, the graph also excludes today's temperature by stopping around the year 1900. For a more complete graph see

Even if they're overstated, the Roman and Medieval Warm Periods do indicate that the climate can go through significant swings.  The RWP and MWP may have been initiated by changes in volcanic activity, solar irradiation or ocean currents, but for today's warming the only serious explanation is greenhouse gases.

As for the wine, there are now more English vineyards than ever.

See also MWP in 2013 IPCC Report

Arctic undersea volcanoes

Researchers have found that volcanoes have been erupting on the Arctic seafloor over the last decade. What effect might this be having on sea ice?

Seafloor vulcanism in the Arctic is not new. What's new here is that it was explosive, like a land volcano. This would have been caused by unusually high levels of volatiles and was not necessarily unusually energetic.

Either way, the amount of heat involved is tiny compared with the observed ice melt.


What Cyclone Increase?

Climate scientists predicted that GW will not increase the number of cyclones, but will make some stronger. In February 2010, scientists at the US National Hurricane Center reported modelling showing that while the worst will indeed be more violent, they will be fewer in number.

One difficulty in monitoring this is that the damage done by cyclones tends to increase over the years simply because more habitation lies in their paths.


Jan 2011: Decades before a clear link can be made to increased storm damage:

Sep 2011: Why hurricanes will be more destructive:

It's the Water Vapour

Denialists point out that water vapour is responsible for 90% of the Earth's greenhouse effect, while CO2 is only responsible for 4%.

This is true at a very simple level, but overlooks the dynamics. If the CO2 increases then the temperature rises a little, but that puts more water vapour into the atmosphere, which boosts the temperature much more than the CO2 increase alone did.

That is exactly why increased CO2 is so dangerous. If it were not for this amplification there would not be a warming problem (but there would still be the problem of too much CO2 in the oceans).

And there's nothing we can do to reduce the water vapour directly. We live on a wet planet, and the amount of water vapour will always be determined by the temperature.

Clouds are a negative feedback

The net effect of clouds is the greatest uncertainty.  The most respectable scientific objections to greenhouse gases constituting a threat claim increased cloud will have a net cooling effect, so limit the rise.  Low level clouds cool by reflecting sunlight while not managing to slow heat loss much; high altitude clouds trap heat much more.

Different climate models make different assumptions about how much extra cloud will form at each level.  Those assuming a net warming effect match the historical data better.

Only 3.4% of CO2 Anthropogenic

Some denialists claim that only 3.4% of atmospheric CO2 is man-made.

The proportion of CO2 in the atmosphere has risen from 280ppm to 390ppm since the start of the industrial revolution, and some of it would have been anthropogenic before that. So that's at least 28% and rising.

Maybe they mean only 3.4% of the annual emissions to the atmosphere are anthropogenic, but that's irrelevant. The non-anthropogenic portion is all part of a long established cycle of emission and absorption. If we add 3.4% to the emissions, some of that will be reabsorbed, but the rest accumulates. It's the accumulation that hurts. Go on long enough and most of what's in the atmosphere will be anthropogenic.


Science is by proof, not consensus

It has been argued that science is not a democracy.  Here, it is put well:

Science is a discipline which relies on testing hypotheses and exposing flaws, (scientifically known as falsification), not on consensus, in order to further scientific understanding. Scientific fact is not a democracy. Scientific facts are not concerned with what the majority of people or scientists think or do not think. The laws of physics are not subject to the democratic vote of a group of scientists; they cannot be repealed by a popular vote. Albert Einstein, for example, when asked to comment on the book One Hundred Authors Against Einstein which denounced his Theory of Relativity, stated that 'to defeat relativity one did not need the word of 100 scientists, just one fact'

What the argument glosses over is that falsification only disproves, it doesn't prove.  Scientists and philosophers mostly accept that no theory (outside the pure reasoning of mathematics) can be proved in any absolute sense.  So how do we acquire new scientific knowledge?  When the substantial majority of qualified scientists accept that adequate attempts to disprove various alternative theories have left one as the winner,   we have consensus.

 It's the Sun

Various ways have been suggested in which changes in the Sun could affect climate.

  • Irradiance

The Sun's brightness has only been measured since 1977.  It got brighter to 1985, but has dimmed ever since.

  • Cosmic Rays

Cosmic rays might affect cloud formation, but repeated attempts to find a correlation have been inconclusive.  CERN is running experiments to look for a possible mechanism anyway.

Oct 2011: CERN finds cosmic rays do assist in forming condensation nuclei; significance for clouds and climate undetermined

Dec 2012: CERN research continues, though relevance to climate change less and less likely

Nov 2013: Less than 10% of 20th century warming due to solar changes

Mar 2015: Cosmic Ray effect small and short-term

  • Sunspots

Sunspot numbers have trended down since 1920, indicating a less active Sun.

Aug 2012: Severity of European winters linked to sunspot cycle

  • Ultra Violet

We only have measurements since 2003, but since the Sun has become less active over the last 20 years it is likely UV has fallen too.

  • Magnetic Field

The Sun's field flips every 11 years.  It may affect weather patterns, though the evidence is not strong, and so contribute to short-term variations in climate.  Perhaps it partly explains the pause in warming from 2001 to 2009, but if so, that weighs against the deniers' case.

So there is no good reason to suppose that the Sun's changes are responsible for an overall warming. If it were, for example, we would expect the stratosphere to have warmed too, but it has cooled.  That's entirely consistent with greenhouse gases trapping heat in the troposphere (the bit where we are) and leaving the stratosphere above it to become cooler.

There is evidence that variations in the Sun shift weather patterns, resulting in a large redistribution of the heat.   In particular, shifts in the jet stream can produce cold winters in Europe.

Jan 2012: Century of weaker sun will not delay GW

Aug 2013: Solar influx explains warm peak in 1930s, but not since

Jan 2014Yet more research discounting solar variation, finding stronger link to GHGs

It's the CFCs

Claim that CFCs caused the warming fail basic analysis

There is no Greenhouse effect - R. W. Wood proved it in 1909

Wood was a fine scientist, but by his own account he did not probe very deeply into the question of atmospheric greenhouse effects:

I do not pretent to have gone very deeply into the matter, and publish this note merely to draw attention to the fact that trapped radiation appears to play but a very small part in the actual cases with which we are familiar.

Here's his whole paper. 

While Wood's experiment does suggest the "greenhouse effect" is not particularly important in how greenhouses get hot during the day, there are major flaws in his experiment:

To test the matter I constructed two enclosures of dead black cardboard, one covered with a glass plate, the other with a plate of rock-salt of equal thickness. The bulb of a themometer was inserted in each enclosure and the whole packed in cotton, with the exception of the transparent plates which were exposed. When exposed to sunlight the temperature rose gradually to 65 oC., the enclosure covered with the salt plate keeping a little ahead of the other, owing to the fact that it transmitted the longer waves from the sun, which were stopped by the glass. In order to eliminate this action the sunlight was first passed through a glass plate.

There was now scarcely a difference of one degree between the temperatures of the two enclosures. The maximum temperature reached was about 55 oC. From what we know about the distribution of energy in the spectrum of the radiation emitted by a body at 55 o, it is clear that the rock-salt plate is capable of transmitting practically all of it, while the glass plate stops it entirely. This shows us that the loss of temperature of the ground by radiation is very small in comparison to the loss by convection, in other words that we gain very little from the circumstance that the radiation is trapped.

In respect of how greenhouses really work, there are two flaws:

  • In his second version, he has overlooked that the extra glass plate above the rock-salt plate will become hot and radiate back down through the rock-salt plate.  Since rock-salt is transparent to IR he should not expect any difference from the glass-without-rock-salt box, regardless of whether the glass produces a "greenhouse effect".
    Better would have been an inverted V roof with one side facing the sun and the other side away, the sunward side being rock-salt in both cases.  When the other side is glass instead of rock salt, greenhouse theory says the inside should be warmer.
  • Wood does not appear to have considered that staying relatively warm at night is at least as important.  At night, the glass will trap heat better than the rock salt.  He should have operated a night-day cycle.

In respect of what the experiments say about the atmosphere, the same two issues arise, and two more:

  • The lightsource in Wood's experiment has already come through the atmosphere.  This means the proportion of its energy that is in the IR band will differ from that for the case of light entering the atmosphere from above.
  • GHGs in the atmosphere can be thought of as many layers, each absorbing some of the IR that tries to pass through it and re-emitting it in a random direction.  So if more IR is coming from the lower layers of the atmosphere than is coming from space, each of these layers will tend to redirect some IR back down. 
    A single glass plate will only model one layer.  On the one hand it will absorb all IR in each direction, on the other it will re-emit at most half back down (some will be lost upwards by air contact and convection).  A better model for the atmosphere would have multiple glass layers with small air gaps.
Even in Wood's day, others failed to reproduce his findings on greenhouses.  There's an excellent history at, and a modern day replication attempt at

There is no Greenhouse effect - it's the Adiabatic Lapse Rate

Thermodynamic theory says that without an atmosphere the Earth's surface would be at -18C, on average.  Greenhouse Theory says that it's warmer than that because some of the heat radiated from the surface is intercepted by gases in the atmosphere, principally water vapour and CO2, and reradiated back down.

The role of water vapour is complicated because the amount of it also depends on the temperature.  Standard climate science says that without CO2's contribution the temperature and atmospheric water vapour would decrease in a downward spiral until the earth were a snowball.  Indeed, this does seem to have happened in the distant past, when the sun was a bit cooler.

But some physicists claim there is no need for a greenhouse theory to explain Earth's balmier average of +14.5C.  E.g. this paper by Joseph Postma.  Here is his argument:

  • He observes that the temperature of -18C occurs at the tropopause, 5km altitude: "This altitude is found at about 5km in height above the ground surface by observation. "
  • He calculates from more thermodynamic theory (the "adiabatic lapse rate" or ALR) that we should therefore expect the temperature at the surface of the earth to be 14.5C. Voila! No need for a greenhouse effect.

 But why is it -18C at 5km height?  He provides no mechanism.

His error is to confuse cause with effect.  The ALR is not a mechanism for transporting heat; the mechanism for that is convection.  The ALR is a threshold value of the temperature gradient.  When the temperature gradient is below this threshold the temperature difference between the layers is not enough to drive convection. 

Such temperature "inversions" occur naturally.  The greenhouse gases at the tropopause cool by radiating into space.  This continues until the temperature gradient below it exceeds the ALR.  This gets convection going, transporting heat up from the lower layers and reducing the gradient again.  So, most of the time, the gradient is about equal to the lapse rate.

So although you can infer a maximum temperature of 14.5 at ground level if you are told it is -18 at 5km, causation runs the other way: it is -18C at 5km because:

a. the GHGs at the tropopause radiate heat away into space, and

b. a ground level temperature of 14.5C will drive convection and stop the tropopause getting any cooler. 

If the ground level temperature rises above that for any reason, convection will increase.  This may increase the temperature at the tropopause, or raise it higher, or some combination of the two. 

With no greenhouse gases (and ignoring the equatorial/polar difference in insolation) it would be -18C all over the Earth's surface and all through the atmosphere.

Another misconception by Postma and others is that the laws of thermodynamics prohibit heat flowing from a colder body to a warmer one.  (This is why they question the greenhouse effect in the first place.)  What those laws actually prohibit is a net flow of heat in that direction.  With only very special exceptions, any body not at absolute zero radiates heat in all directions.  If some of that radiation hits a warmer body then it can warm it further.  But the warmer body will radiate even more heat the other way.  In this way, a relatively cold atmosphere can help to warm the Earth's surface.

Sea level rise is decelerating around Australia

There simply is not enough data to say whether it is accelerating, decelerating or remaining steady.  The report from NSW government's Phil Watson ( is based on the only four sites around Australasia where the records go back a century. 

They all show an overall rise, though at differing rates, and the variations from the local average rates occur at different times on different coasts.  In fact, the overall tendency is that when it rises faster than usual on our West coast it's slower than usual on the East, and vice versa.  So these data say more about local decadal variations than about any overall acceleration or deceleration.

 That said, there is a hint of deceleration at the global level:  But a very similar pause occurred in the mid-1990s, and it might be explained by the recent La Nina.

The temperatures correlate with <put your favourite non-GHG variable in here>

A pleasant sceptic pastime is to find a correlation between temperature records and some natural variable such as solar cycles, planetary orbits, Earth's wobble, whatever.  The sheer number of possibilities is the problem: it means you're likely to find a correlation with something, even if there is no physical connection.  In the absence of a reasonable mechanism, the correlation would have to be extremely good to carry any weight.

For a neat parody, see