By Andy May
It’s more likely mostly due to both, but that isn’t really the question. Virtually everyone accepts that climate changes and that CO2 and methane are greenhouse gases; and probably everyone remembers from grade school that the Sun is a variable star. The debate is over how much of recent global warming is due to the Sun, either its internal variability or changes in the Earth’s orbit, and how much is due to human greenhouse gas emissions, mostly from fossil fuels?
The role of the oceans
Global short-term weather patterns (<30 years) are driven by the oceans, especially ocean oscillations like the El Nino/Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO) (NOAA 2018). Some portion of longer-term climate changes (>100 years) may be driven by the thermohaline circulation (THC). This is a process where surface ocean water becomes dense enough to plunge deeper in the ocean and join the meridional overturning circulation (MOC) currents that carry deep water around the world. This exchange of surface for deep and later deep for surface waters mainly occurs in the polar regions, where deep water is ventilated and equilibrates with the atmosphere. Probably the only exception to polar overturning is in the mostly isolated Mediterranean Sea. The driver for the overall THC is high latitude cooling of sea water, although salinity also plays a role. This slow set of deep ocean currents contains over half the water in the oceans and has a temperature of less than 4°C due to its ties to the polar oceans. The currents complete a worldwide trip in a thousand years or more (Rahmstorf 2006). A diagram of the process in the Atlantic is shown in figure 1 from Rahmstorf, 2006. The speed of the THC and AMOC varies and climate varies with it.
Another process, ocean CO2 uptake, may also contribute to longer term climate changes. The solubility of CO2 in water increases as the water temperature drops. When temperatures drop, as they did in the Little Ice Age, more atmospheric CO2 is taken up by the ocean and this accelerates the temperature drop. The reverse occurs when temperatures rise, as they have recently.
Still another contributor to long-term natural climate variability are the longer tidal cycles (Berger and Rad 2002). These tidal cycles disrupt the ocean stratification, allowing warm water surges under sea-ice that cause the ice to melt from the bottom. In glacial periods, this melting results in a release of considerable thermal energy since warm water trapped under the ice is ventilated. The effect on the climate is a function of the original extent of the sea ice, so it is more noticeable during glacial periods. The effect of the vertical tidal mixing of the oceans is less predictable in interglacial periods. The basic unit of the lunisolar tidal cycle is 375 years and the largest impact is seen every 1,500 years or every fourth beat of the cycle. The cycle is caused by the Moon’s nodal and apsidal orbital precession. For more discussion of this see the cited article by Berger and von Rad or Javier’s post: Nature Unbound V.
The world ocean contains 99.9% of the surface thermal energy on the Earth, the atmosphere contains 0.07% of the energy. Thus, while atmospheric processes often dominate the weather over short periods of time (2 weeks or so), the climate is dominated by the oceans. While oceans drive our climate, what drives changes in the oceans? They have no interior energy source. They collect most of the solar energy that makes its way to the Earth’s surface, as well as most of the thermal energy radiated toward the Earth by atmospheric greenhouse gases. Except for cosmic rays, a little thermal energy supplied by submarine volcanos and the occasional bolide (meteor) impact, that’s it.
The two hypotheses
So, we will look at two hypotheses that can supply the energy necessary to drive ocean and climate changes. In this post we discuss the greenhouse gas hypothesis. In this hypothesis the concentration of greenhouse gases, or more precisely “Infrared active gases” in the atmosphere, especially carbon dioxide, control the climate. As carbon dioxide is added to the atmosphere, through burning fossil fuels, the atmospheric temperature goes up and this increases the specific humidity of the atmosphere through the Clausius-Clapeyron relationship. The added water vapor acts as a positive feedback and the temperature goes even higher. Specific humidity or absolute humidity is the volume fraction of the atmosphere that is water vapor.
The IPCC AR5 WG1 Physical Science Basis Report calls CO2 the “main anthropogenic control knob on climate” on page 667 (IPCC 2013). Although water vapor is more abundant in the atmosphere and a more powerful greenhouse gas, it is considered less important than CO2 since, under this hypothesis, the temperature of the air is primarily controlled by the fraction of CO2 and other, less important, noncondensable, infrared active gases like methane. This is discussed in the FAQ 8.1 on pages 666-667 in the IPCC AR5 WG1 report (IPCC 2013).
In the next post (Part B) we will discuss the solar variability hypothesis which contends that solar variability controls the climate. There are several things that can change the radiation we receive significantly. The Earth’s orbit and tilt relative to the orbital plane varies over time and these factors change the amount of solar radiation striking the Earth and where it strikes on the Earth, this is explained well in Javier’s post “Nature Unbound III; Holocene climate variability (Part A)” (Javier 2017). Figure 2 graphically shows the components of the Earth’s orbit.
The strength of the Earth’s magnetic field and the solar magnetic field change with time and they interact in complex ways that are not fully understood, but these magnetic fields control the number of cosmic rays that strike the Earth. This affects our weather (Svensmark, et al. 2017) (J. Svensmark, et al. 2016) and maybe our climate by increasing or decreasing cloudiness. The number of cosmic rays striking the atmosphere is recorded in the berylium-10 and carbon-14 cosmogenic isotope record. Berylium-10 (10Be) is stored in ice cores and carbon-14 (14C) is stored in tree rings.
The number of sunspots has been found to correlate with both cosmic ray created (cosmogenic) isotopes (10Be and 14C) and with historical climate changes, as well as global temperature proxies, such as glacial advances, biological temperature proxies and oxygen-18 (18O) temperature proxies. Thus, it is assumed that sunspot records provide a long-term measure of some significant component of solar activity. It isn’t clear exactly what the number of sunspots means, but whatever it is, it does seem to correlate, to some degree, with climate.
Which hypothesis dominates climate change? Do both act on the climate equally? Are there other processes that are important? The debate on climate change is about “how much.” How much is natural or based on the amount of solar radiation striking the Earth, discussed in Part B of this series? In this post we discuss what we know about how much is due to humans burning fossil fuels and extracting or making methane gas. While researching these posts, we found that we really don’t know much about “how much.”
The Greenhouse Gas Effect (or CO2 effect) hypothesis
There are many definitions of the “greenhouse effect” (GHE) and they often conflict with one another when examined in detail. Even the more specific phrases, the “CO2 GHE” or the “CO2 enhanced GHE” are frequently conflated with each other, causing confusion. It has been asserted, by the IPCC and others, that the CO2 GHE might be dangerous. How much evidence is there to support this assertion?
Douglas Fischer proposed in Scientific American (Fischer 2009) that man’s emissions of carbon dioxide, from burning fossil fuels, could eventually cause the whole planet to warm to a dangerous temperature. Fischer writes:
“[Global climate] models do suggest that failure to stem industrial exhaust will push global temperatures four degrees Fahrenheit above today’s readings – well beyond a threshold many scientists fear will produce dreadful consequences.”
The U.S. EPA stated in a press release December 7, 2009 (EPA 2009):
“… U.S. Environmental Protection Agency (EPA) announced today that greenhouse gases (GHGs) threaten the public health and welfare of the American people. EPA also finds that GHG emissions from on-road vehicles contribute to that threat.
GHGs are the primary driver of climate change, which can lead to hotter, longer heat waves that threaten the health of the sick, poor or elderly; increases in ground-level ozone pollution linked to asthma and other respiratory illnesses; as well as other threats to the health and welfare of Americans.”
The EPA calls this their “endangerment finding” and they list six key greenhouse gases, carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride that they consider dangerous because they act to warm the atmosphere. More details on their finding are given in an action report (EPA 2010).
James Hansen (NASA, as quoted in Fischer’s Scientific American article cited above) once said “We are now completely in charge” of the climate. He also said, “We’re much more powerful than the natural forces.” Just like the IPCC (see above), Lacis, et al. have written in the journal Science that atmospheric CO2 is the principal control knob for climate change (Lacis, et al. 2010). As we will see, the impact of carbon dioxide on the Earth’s climate has never been measured in nature. This is true despite numerous attempts. The estimated range of the impact of doubling CO2 (the “ECS” or equilibrium climate sensitivity) remains at 1.5° to 4.5°C, precisely where it was in the Charney Report in 1979 (Curry 2017). The statements by Hansen and Lacis, et al. and the EPA’s proclamation are based on the UN IPCC fourth (IPCC 2007) and fifth assessment (IPCC 2014) reports and computer climate model results (Johnson 2009).
Most scientists who study climate look to the UN IPCC as a source of well-reviewed data and analysis, so we will discuss their definition of the “greenhouse effect.” In the glossary of their fifth report they define it as follows:
“The infrared radiative effect of all infrared-absorbing constituents in the atmosphere. Greenhouse gases, clouds, and (to a small extent) aerosols absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere. These substances emit infrared radiation in all directions, but, everything else being equal, the net amount emitted to space is normally less than would have been emitted in the absence of these absorbers because of the decline of temperature with altitude in the troposphere and the consequent weakening of emission. An increase in the concentration of greenhouse gases increases the magnitude of this effect; the difference is sometimes called the enhanced greenhouse effect. The change in a greenhouse gas concentration because of anthropogenic emissions contributes to an instantaneous radiative forcing. Surface temperature and troposphere warm in response to this forcing, gradually restoring the radiative balance at the top of the atmosphere.”
Thus, the IPCC divides the definition into two parts. The “greenhouse effect” is the impact of greenhouse gases on surface temperature in general, an effect that has been around for many hundreds of millions of years, and the “enhanced greenhouse effect” is the additional effect caused by increasing the concentration of greenhouse gases, particularly CO2. They do not discuss, in this definition, any other mechanisms for increasing the average surface temperature of the Earth. Water vapor is a greenhouse gas and can store thermal energy for a long time as latent energy (heat). Carbon dioxide is a noncondensing gas at normal temperatures and immediately radiates any energy it absorbs. That energy will warm a nearby body, via heat transfer, that is at a lower temperature. The radiated energy can excite other molecules causing work to be performed through wind or current motion, this movement of mass can potentially add or subtract thermal energy in an object, or the surface of the Earth. The net effect of additional greenhouse gases is to slow the natural cooling of the surface of the Earth to outer space.
Another quote, from Lacis, et al., takes the IPCC AR5 line of reasoning a little further (Lacis, et al. 2010):
“The difference between the nominal global mean surface temperature (TS =288K [15°C]) and the global mean effective temperature (TE= 255K [-18°C]) is a common measure of the terrestrial greenhouse effect (GT = TS – TE = 33 K). Assuming global energy balance, TE is also the Planck radiation equivalent of the 240 W/m2 of global mean solar radiation absorbed by Earth.”
The temperature Lacis, et al. call “TE” is the same as the calculated blackbody temperature of the Earth. If we assume the Earth is a blackbody, then the Earth emits as much radiation as it receives and is a perfect energy absorber. This also means we are assuming the Earth’s surface has a constant temperature and is at thermal equilibrium with its surroundings. Thus, Lacis, et al.’s TE of 255K, assumes that the Earth is a blackbody that emits and receives 240 W/m2, in the absence of greenhouse gases. The Earth is not a blackbody, it is colored (or a “gray body”), and the atmosphere is a different gray body. The Earth is also a rotating sphere that has circulating oceans that contain most of the thermal energy stored on the surface. While the blackbody assumption is flawed, it is commonly used as a device to “compute” the greenhouse effect. This contrivance is mainly used to pretend we know what the greenhouse effect is and its magnitude, however nothing could be farther from the truth.
Equilibrium Climate Sensitivity (ECS) to a doubling of CO2
It has been useful to boil down the effect of increasing the atmospheric carbon dioxide concentration to a single number. The equilibrium climate sensitivity (ECS) is the temperature response to a sudden doubling of carbon dioxide concentration after the oceans have reached equilibrium with the new atmospheric temperature. There are several ways to compute this. The IPCC, as reported in AR5 (Chapter 10, figure 10.1, page 879), prefers to compare two climate model computer runs (IPCC 2013). One model run applies only to estimated natural climate forcing and the other adds an estimated forcing from human-generated carbon dioxide emissions and other human activities. They take the difference in the final modeled temperature after 150 years as ECS. Figure 3 illustrates the IPCC calculation of the effect of human greenhouse gas emissions, mainly CO2. It is entirely model-based and makes many assumptions; the most significant assumption is that the only natural climate forces are their estimate of total solar irradiance (TSI) and volcanic eruptions for the period shown (Lewis and Curry 2015).
The IPCC estimate of natural climate does not include the impact of longer ocean oscillations such as the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), and it does not model the El Nino/Southern Oscillation (ENSO) successfully. These oscillations in ocean surface temperature can have long time frames, up to 60-65 years (see Javier’s post here for a discussion of this), and the AMO has distinct peaks around 1875, 1940, and 2005; the latter peak could account for some of the warming shown in Figure 3. It is instructive that the global warming experienced between 1910 and 1940 is not successfully modeled by either the CMIP3 (blue) or the CMIP5 (red) model ensembles, see Figure 4. For more on the poor model fit of this period, see here.
Another potential problem with the IPCC natural variability estimate is the considerable uncertainty in the amount of thermal energy (heat) that is absorbed by the oceans, often called ocean heat uptake (OHU) (Lewis and Curry 2015). However, regardless of these uncertainties, the IPCC AR5 WG1 report estimates that natural variability (NAT) and internal natural climate variability are zero with low uncertainty as shown in Figure 5.
Proistosescue and Huybers, report in Science Advances that observationally based estimates of ECS based on historical instrument records (which only go back to 1850 or so) fall in the range of 1.5° to 3°C, much lower than estimates from global climate models (GCM) (Proistosescu and Huybers 2017). They include the estimate of 1.5° to 4.5°C by the IPCC. Proistosescue and Huybers write that currently the world is not at climate equilibrium and is taking up between 0.1 and 0.9 W/m2 more thermal energy than it loses. As a result, they do not believe that observationally based estimates of ECS are valid. They used a group of GCM simulations to compute an ECS range of 2.2° to 6.1°C, with a most likely ECS of 3.5°C per doubling of CO2. This technique has the same problems as the one used by the IPCC in that it assumes the natural climate forcings and sources of internal variability are known and accounted for accurately. It also assumes the precise impact of carbon dioxide and the carbon dioxide feedbacks are known. They are saying, in effect, ignore the data and use my computer model.
An example of an observationally based estimate of ECS is the work of Nic Lewis and Judith Curry, published in Climate Dynamics (Lewis and Curry 2015) where they derived a tight cluster of ECS estimates from 1.64° to 1.72°C per doubling of CO2. Lewis and Curry later updated this study in (Lewis and Curry 2018) where they derived an ECS of 1.5°C to 1.67°C. These estimates are close to the estimates by Otto, et al. that range from 1.91° to 2.0°C (Otto, et al. 2013). The base period 5-95% uncertainty estimate from Lewis and Curry is 1.05° to 2.45°C. The periods they used for the calculation are 1869-1882 and 2007-2016. Thus, these estimates account for the shorter ocean oscillations, but not the longer “internal variability” ocean oscillations involving the deep ocean waters, such as the ~1,500-year cycle described by Debret, et al. in Quaternary Science Reviews (Debret, et al. 2009). The cycle Debret, et al. describe may be due, in part, to the thermohaline circulation; but this is still controversial. Figure 6 is from Otto, et al. and plots the observational constraints on ECS using data from various recent decades. It shows that recent temperature changes and measured carbon dioxide increases suggest an ECS of around 2°C. This rough calculation was expanded and refined by Lewis and Curry in 2015 and in 2018.
Other researchers have used satellite data to estimate the CO2 climate feedback factor and assume a direct CO2 forcing of 1.1°C per doubling of CO2, from laboratory measurements, absent any feedbacks. This was done by Lindzen and Choi who reported an ECS of 0.7°C (0.5° to 1.3° 99% confidence interval) in the Asia-Pacific Journal of Atmospheric Sciences using this technique (Lindzen and Choi 2011).
All these methods make many assumptions and use models that can be challenged. However, the pure computer model approach used by the IPCC and Proistosescue and Huybers seems to be the weakest to this author and their estimates stand alone at the very high end of recent estimates. The more direct observationally-based methods like those used by Lewis and Curry, Lindzen and Choi, and other similar estimates all produce lower estimates of ECS. While it is true, as written by Proistosescue and Huybers, that the climate today is not at equilibrium, I doubt it has ever been in equilibrium over periods shorter than 60 years (for example 1951-2010), except by accident. In any case, model estimates cannot and should not be taken over direct measurements. Recent reasonable estimates of ECS, based on measurements, range from 0.7° to 2°C, this suggests two things. First, we have no reliable estimate of how much our fossil fuel CO2 is affecting the climate; and second, observations suggest that the impact of the additional CO2 will not be a problem. It is possible to create a model that will make it appear to be a problem, but that is not cause for alarm.
As I wrote in my new book Climate Catastrophe! Science or Science Fiction?
“As mentioned above … 99.9 percent of the Earth’s surface heat capacity is in the oceans and less than 0.1 percent is in the atmosphere. Further, CO2 is only 0.04 percent of the atmosphere. It beggars belief that a trace gas (CO2), in an atmosphere that itself contains only a trace amount of the total thermal energy on the surface of the Earth, can control the climate of the Earth. This is not the tail wagging the dog, this is a flea on the tail of the dog wagging the dog. Extraordinary evidence is needed to convince us of this hypothesis. Since the impact of man-made CO2 on climate has never been measured and is only crudely estimated with unvalidated models, the jury is still out on this idea.”
In the next post, “Part B,” we will discuss the impact of solar variations on the climate.
The bibliography can be downloaded here.
The data used to make many of the figures in this post can be downloaded here.
Javier provided many helpful comments on this post.