What is warming the Earth?

Guest Post By Javier

A no-assumptions look at the global warming evidence helps clarify the possibilities.

The planet’s surface has been warming since the depths of the Little Ice Age, and particularly since ~ 1850 AD. The surface temperature record, however incomplete or uncertain, reflects this warming. Hypotheses about why the warming is taking place can be grouped into three general categories:

1. The energy input is increasing. This is the basis of the variable solar output hypothesis.
2. The energy output is decreasing. This is the basis of the greenhouse gases hypothesis.
3. The transfer of heat within the system is changing. This is the basis of some hypotheses for a reduced vertical exchange in the ocean, or for changes in the oceanic currents that redistribute the heat.

A combination of these categories cannot be ruled out.

Whatever causes the temperature change must necessarily affect its rate of change, the velocity at which temperature changes over time, measured in °C/decade. A velocity that varies continuously and can be positive (warming) or negative (cooling).

Figure 1. 9-year global surface temperature rate of change (4-year averaged) in
°C/decade. The Pause is indicated by the khaki box. Source: Met Office UK, HadCRUT 4.

There are two features in the evolution of the temperature rate of change since 1850 (figure 1).

1. A long-term increasing trend, represented by the linear adjustment, that indicates the warming has been progressively accelerating. The current rate is 0.14°C/decade.
2. A very variable rate that indicates that the surface of the planet does not warm continuously but through periods of warming interrupted by periods of cooling.

These two features have been described with the analogy of a man walking his dog. The man’s path being the long-term trend, and the dog’s path the variability around that trend. There is an apparent but irregular periodicity in the temperature rate of change, meaning that the dog does not move at random. Periods of warming and cooling are of roughly 30 years each, constituting the known 60-year oscillation.

Additionally, during each 30-year period there is a reversal in the direction of temperature change, that goes through a period of change in the rate opposite to the main oscillation. This behavior of the data can be graphically represented by a curve oscillating with these characteristics (Figure 2, thick grey curve).

Figure 2. 9-year global surface temperature rate of change (black curve) and 60-year oscillation (grey curve). The analogous position to the present in previous oscillations is shown with red arrows.

What we call the “Pause” is just the latest change in the periodical behavior of the temperature rate of change. Something that should be expected simply as an extrapolation of past behavior. However, assumptions taken about the cause of global warming precluded most scientists from simply extrapolating past observations. Further extrapolation suggests the Pause is the beginning of a non-warming period that should be ~ 30 years long.

The examination of the evidence without assumptions leads to the following observations:

• The surface of the planet has been warming in a linearly accelerating way since 1850. The long-term rate of warming is currently 0.14°C/decade.
• Periods of higher rate of warming alternate with periods of lower or negative rate of warming in an irregular ~ 60-year oscillation.
• The rate of warming presents also a ~ 20-year oscillation, and usually inverts its direction of change after about a decade.

Now we can start testing possible causes. The leading candidate according to most climate scientists is the anthropogenic increase in GHGs. By comparing the temperature rate of change with the increase in the main GHG, CO₂, we can see that this affirmation is not supported by the evidence (figure 3).

Figure 3. 9-year global surface temperature rate of change (black curve, LHS) and 10-year change in the natural logarithm of atmospheric CO₂ concentration (blue curve, RHS). The logarithm better represents the changes in forcing by CO₂. The red dashed lines are the linear adjustment to the black curve for each half of the data. Source: 1850-1958 Law Dome. 1959-2017 NOAA.

We can see why many scientists are mistaken about this issue. The increase in CO₂ since the 1960s coincides with a period of increase in temperature rate of change (orange box in figure 3). They just needed to explain away the prior cooling (blue box in figure 3) which they did by using aerosols, a byproduct of the early global
industrialization. However, by looking at the long-term rate of change we can see that the increase in CO₂ cannot explain the increase in warming from 1850-2018. Most of the CO₂ increase has taken place in the second half of the period, while both halves of the temperature data are not significantly different and have a similar slope in their linear adjusted rate of change (dashed red lines, figure 3).

Obviously, the increase in CO₂ cannot explain the 60-year oscillation either. This oscillation is also unlikely to have a solar origin, as there is no 60-year solar periodicity. It may have an oceanic origin, since it is present in some oceanic indices. Alternatively, it could be caused by an oceanic-atmospheric interaction.

Solar variability can be related to both the long-term increase and the periodical oscillation in the temperature rate of change (figure 4). It is at least plausible that it contributes to both.

Figure 4. 9-year global surface temperature rate of change (black curve, LHS) and 11.1-year averaged monthly sunspot number (red curve, RHS). The position of the lows from two well known solar periodicities are indicated by red arrows. Source: Silso.

Measuring solar activity by its proxy, the 11-year averaged monthly sunspot number, we can see a long-term increase in solar activity since 1850, matching the increase in rate of warming. Additionally, the oscillations in solar activity are compatible with the oscillations in the temperature rate of change, particularly during the first 80 years of the record and the last 40. However, the 40 years in between are in clear disagreement.

Periods of very low solar activity in the 170-year record coincide with periods of low or negative temperature rate of change. It is possible that the 60-year oceanic oscillation is paced and reinforced by the periods of low solar activity and the oscillation then continues through the periods of higher solar activity.

The CO₂-hypothesis is an atmospheric hypothesis of climate, where the atmosphere acts as the main controller of how much energy enters or leaves the system. Its main contender is the oceanic-solar hypothesis of climate, where the oceans control the surface temperature and do so by integrating the changes in solar output and deep ocean heat exchange. In this hypothesis the oceans are quite sensitive to small solar changes but also to other factors (clouds, wind, upwelling) that can determine a different response at times. The climate control by the oceans
could compensate, in great measure, for changes in atmospheric non-condensing greenhouse gases, like CO₂. This would explain the thermal homeostasis of the planet during the Phanerozoic Eon, when great changes in solar output and GHGs took place.

In conclusion, a no-assumptions look at the evidence of warming shows that solar forcing has changed during the period 1850-2018 in a more consistent manner with the warming rate than CO₂, and thus constitutes a better candidate for the main cause of the observed warming. There is insufficient evidence to evaluate other possibilities over the entire period.