Temperature changes are caused by changes in the Earth’s orbit, not by CO2 Fact: Over many millennia, the Earth’s temperature changes significantly. Those changes are set off by variations in the planet’s orbit, tilt and axial rotation (all of which are affected, in turn, by the gravitational pull of the larger planets). The change from an ice age to a thaw and back again is known as a Milankovitch Cycle. What happens is that once every 100,000 years the Earth’s orbit, tilt and rotation line up in such a way as to significantly change the amount of solar radiation reaching the planet. This in turn sets off a feedback loop that induces an ice age or a thaw. It takes between 5,000 and 20,000 years for the Earth’s temperature to make the change. It then remains stable until the next correlation of orbit, tilt and rotation occurs. The Earth emerged from the last ice age around 12,000 years ago and is not due to enter another for about 70,000 years.  Milankovitch Cycles are not the cause of our current global warming.   Ice ages and thaws over the last 800,000 years [Source: Wikimedia Commons]

Recent temperature rises have been caused by solar activity, not increases in CO2. Fact: Solar radiation reaching the Earth (known as insolation) can only result in minor temperature changes. The sun’s energy fluctuates on an 11-year cycle. It only changes by about 0.1 percent each cycle, so its impact is very small.  Scientists estimate that solar activity has contributed about 0.1°C to temperature increases since 1870. For the last three decades solar activity has actually been decreasing, yet the planet isn’t cooling. If CO2 levels had no impact on global temperatures, as climate sceptics claim, then you would expect this not to be the case. What is actually happening is that the impact of CO2 emissions is overwhelming any cooling effect from decreased insolation. Of course, when solar activity starts to increase again, it will exacerbate the impact of greenhouse gas emissions.  Another important factor is that if solar activity were causing recent temperature increases, the atmosphere would warm uniformly. In fact, the upper atmosphere (above 20 kms) is cooling, while the lower atmosphere is warming. This is not consistent with solar activity. It is consistent with increases in CO2, however. In addition, if solar activity were the cause of recent warming, summers would be warming more than winters, and days more than nights. In actuality, neither of these things is happening.  Hence, all the evidence suggests that CO2 is the cause of the warming we are currently experiencing. Graph showing the disconnect between solar activity and global temperatures over the past half century. [Source: Sceptical Science

We can’t possibly know what temperatures were like centuries ago, let alone millions of years ago. They might actually have been hotter during the Medieval Warm Period (900-1150 AD) than now, so we can’t say the Earth is heating up unnaturally.  Fact: Scientists use ‘proxies’ to calculate past temperatures. These include tree rings, ice cores, coral sediments and sea levels. [The study of past climates is called paleoclimatology.] The data produced by these proxies is not perfectly accurate but does provide a good picture of the past. That data suggests that the Medieval Warm Period was not warmer than today, although temperatures may have been this warm in parts of the northern hemisphere. [Interestingly, climate sceptics are happy to use evidence from proxies when they confirm their views, but not when they contradict them.] For more detail on the Medieval Warm Period, see the previous two myths.

The Medieval Warm Period (MWP) was warmer than today, as evidenced by the fact that the Vikings were able to cross the north Atlantic and settle in Iceland and Greenland due to the reduced ice cover. This is proof that global temperature changes are natural, and that current warming is nothing to worry about.  Fact: The MWP (900-1150 AD) was not warmer than today, although in northern Europe and the North Atlantic temperatures were similar to today’s. In some parts of the world, however, temperatures actually got colder during the MWP. Scientists estimate that on average, global temperatures were similar to those in the mid-20th century. Interestingly, sea levels then were nowhere near as high as they are today. (They rose by 24 cm over a period of 400 years but never got to 20th century levels.)  If we only look at the natural factors that generated the MWP (changes in solar and volcanic activity, changes in oceanic currents and small changes in the Earth’s orbit), there should have been global cooling over the past 50 years, not warming. Yet we are experiencing the opposite.  The current warming trend is unprecedented – 1.2°C in just over a century. We have not seen such rapid warming in the past two million years, and maybe not for 55 million years. During thaws from ice ages, it took between 1,000 and 2,000 years for the atmosphere to warm by 1°C.  It should also be noted that while the MWP improved living conditions in places like Greenland, it devastated regions like the southwest of the United States, which experienced severe and prolonged droughts. Communities in the latter regions were forced to move, while the Vikings were forced to abandon Greenland when the MWP ended. [Had the Vikings discarded their European farming practices and adopted the Inuit lifestyle, they would not have had to leave Greenland.] A final point of interest is that Iceland was settled before the Medieval Warm period began – evidence that reduced ice cover was not the reason the Vikings were able to cross the North Atlantic.

Changes in global temperature are natural. They happen in cycles, and the current warming is no different. The Earth has passed though countless ice ages and thaws. More recently we have seen the Medieval Warm Period (900-1150 AD) and the Little Ice Age (1450-1850 AD), both of which occurred without any significant CO2 emissions by humans.  Fact:  Large temperature changes are natural over many millennia (ice ages and thaws), as are small changes over many centuries. We know what causes these changes, and those factors do not apply at present.  The current level of warming is unprecedented – 1.2°C in a century and 0.65°C in the past 30 years! During a thaw from an ice age, it takes between 1,000 and 2,000 years for the atmosphere to warm by 1.0°C. This means the Earth is currently warming at 33 times the natural rate. There is absolutely nothing natural about this! As far as the Medieval Warm Period and the Little Ice Age are concerned, they involved significant temperature changes in northern Europe and the Atlantic, but not in other parts of the planet. Scientists estimate that the average global temperature changed by between 0.2°C and 0.6°C from the height of the Medieval Warm Period to the lowest point of the Little Ice Age. During the latter period, winters in Europe were colder than today, but summers were not significantly so. There is little evidence of a widespread breakdown of food production.  It should also be noted that the Little Ice Age was not actually an ice age, or anything like it. Many climate sceptics are obsessed with it simply because of its name. They seem to think that because it’s been dubbed an ‘ice age’, it actually was one.  The temperature variance between the Medieval Warm Period and the Little Ice Age [Source: Wikimedia Commons]