Natural cycles cannot be the cause of observed global warming because they don't fit the observed

In a temperature record stretching back into the late 19th Century, the ten hottest years have all occurred since 2010. The hottest by a large margin (at the time of writing - early 2024) was 2023, with 2016 in second place. In both cases manmade global warming augmented by El Nino nudged these years into pole position. The opposite to El Nino, La Nina, is a phenomenon that cools the planet. One of the top ten, 2022, was also the warmest La Nina year on record. Starting to see a pattern here?

There are many natural cycles out there that do affect the climate. Consider the Milankovitch orbital cycles that are strong enough to trigger the switches between glacials and interglacials. These cycles operate over tens of thousands of years so their year-to-year effects are barely discernible.Yet they can cause ice-sheets to wax and wane over vast areas of the planet, especially in the Northern Hemisphere where the vast majority of landmasses currently reside. At the other end of the spectrum is the El Nino Southern Oscillation (ENSO) that most folk have heard of because it causes newsworthy weather events. Climate scientists know all about these cycles and their effects. It's part of the job description.

Those of you who click on the myth's link will find a lot about a cycle known as the 'Pacific Decadal Oscillation' (PDO). That's not a regular cycle that turns up on time, as buses and trains ought to. But yes, it does influence climate as it has warm and cool phases, just like ENSO but in a different part of the Pacific Ocean and over longer periods. And yes, climate scientists monitor the PDO, just like everything else. The PDO is expressed as an Index: values above 0 are positive (warm) and those below 0 are negative (cool). And here's the rub. Since autumn 2019, the PDO Index has been negative, often strongly so. Yet the planet's temperature continues to rise unchecked.

The problem is that in climate more than one thing can happen at once. And since 1950, our CO₂ emissions have surged ever-upwards and the climate is responding to that, too. In other words, human-caused global warming is now overdubbing the effects of such cycles. They used to count for a lot more than they do now.

The carbon cycle describes the way in which carbon moves around the planetary system comprising the atmosphere, the oceans, the biosphere and the solid Earth. The first and last components are where the problem lies. In burning fossil fuels, we have accessed carbon that by rights should have stayed in the solid Earth for untold millions of years. In doing so, that carbon has been dumped into the atmosphere. It represents a disturbance to the carbon cycle rarely seen in the geological record. And the planet is responding to that by heating up.

There's only one cycle we need to worry about and that's the carbon cycle.

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Further details

Global warming refers to the long-term increase in global temperature since the 1850s. The only plausible mechanism for global warming is a planetary energy imbalance. In other words, our climate system is accumulating heat. Let's take a look.

Fossil fuel burning is causing recent global warming

Two factors drive any global climate change. One is internal variability and the other is external forcings. Internal variability refers to the processes moving heat around within the climate system. External forcings differ because they cause an energy imbalance, in which heat is either added to or removed from the climate system. They can have both natural and human causes. In the case of the current warming, scientists have long since concluded that the main cause is the increase of atmospheric CO₂ due to fossil fuel burning. That's been known since the mid 1950s, following decades of investigations.

Internal variability moves heat around the climate system

Global warming has been likened to the motion of water in a kitchen sink. There's an inflow from the tap and an outflow through the drain. The motion has two components. Firstly, there's the water sloshing around. That's what scientists call 'internal variability'. Secondly, there's the increase or decrease in water volume, depending on the relative amounts of inflow and outflow. That's what scientists call 'external forcings'.

Internal variability includes important oceanic cycles. Examples are the El Niño Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO). These well-understood phenomena move heat around, causing regional climatic fluctuations. Such regional effects occur on timescales ranging from years to decades. But like the water sloshing around in the sink, internal variability doesn't add to or take away the total amount of heat in the climate system.

The single cause fallacy committed by the natural cycle myth

This myth, from Don Easterbrook, contended that recent global warming is caused by natural cycles rather than human activity. The fact that it was put forward in 2008 and ten of the warmest years on record have happened since 2010 rebuts Easterbrook's contention all on its own. Easterbrook fell foul of the 'single cause fallacy', by proposing there is only one driver of climate change (e.g. natural factors) when there may be others (e.g. human activity). In fact, there are always many things going on at once when it comes to climate.

Various variants of this myth are out there in the wild, unsurprisingly. Sometimes a particular cycle is picked but quite often it's just an arm-waving 'natural cycle', hoping the reader/listener/viewer doesn't ask for specifics. The best cycles of all, if you want to pretend to your layperson audience that climate change has nothing to do with us humans, are imaginary ones of medium to long duration. These have the great advantage that, for the denier, they will not repeat during their lifetime - most deniers are men who are past to well-past middle age. Ideally, therefore, they want such a cycle to repeat every 50-100 years, so if they are shown to be 100% wrong, natural processes will have ensured they are not around to face the music.

Easterbrook did focus on a specific cycle, the PDO. The Pacific Decadal Oscillation is an irregular cyclical variation in sea surface temperature anomalies over the North Pacific Ocean. It influences weather patterns, importantly including precipitation, across large parts of Asia and North America. That gives us grist to grind because, as with all known cycles, it has been studied in detail and is monitored. The data gathered mean we can index the state of the PDO and plot negative - or cool - phases and positive - or warm - ones against time, just as we do with temperature and other parameters (fig.1).

Fig. 1: the Pacific Decadal Oscillation Index from 1864 to October 2023. Even eyeballing this plot suggests the cool and warm phases more or less cancel one another out. Source: NOAA.

In another in-depth Skeptical Science piece purely about the PDO, ironically dating to around the time that Easterbrook made his claim, the truth was revealed when the trends of global temperature and the PDO Index were plotted on the same graph (fig. 2).

Figure 2: Pacific Decadal Oscillation index (blue - University of Washington) versus Global Temperature Anomaly (Red - GISS Temp). Smoothed data (thicker blue and red lines) and trend lines (thick straight line) are added.

Since then of course, global temperatures have continued to rise with 2023 having been the hottest year on record. Natural oscillations like the PDO don't have the ability to either create or retain heat. They simply move heat around from oceans to air and vice-versa. That means they are not capable of causing a long-term warming trend like the one we are witnessing. When you think about it, if the PDO (or some other internal cycle) was responsible for warming the surface, the oceans would be cooling, which is not the case.

Before the 19th Century, human influence on climate was a fraction of what it is now. Natural cycles were the dominant drivers of global temperature changes. But now, that record has been overwritten. The colossal quantities of carbon released from the solid Earth through fossil fuel burning have made certain of that. Natural cycles can still explain the noise along the upwards curve that is human-caused global warming. However, they utterly fail to explain the continuously upward trend itself. The cause of that is the 50% increase in CO₂ in Earth's atmosphere and that's mostly down to us.

Updated on 2024-03-17 by John Mason.

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