Problems With The Pliocene
Climate alarmists are constantly warning that Earth is going to warm up, driven they say by the level of CO2 in the atmosphere. To bolster their claims they point to the Pliocene, a time 4-5 million years ago, when the planet was 4-8°C hotter and CO2 levels were 400ppm or higher. This is the climate we are heading for, the global warming supporters say—but it that really true? Superficially it seems a plausible assertion, but as it turns out there is much more here than CO2 and temperature. It is not just the average temperature but the distribution of temperature at different latitudes, both over land and sea, that controls the climate. It is the temperature gradient that drives storms and affects weather patterns and it was much different during the Pliocene. Moreover, climate models do not generate a Pliocene like climate when run with higher CO2 levels, which means climate scientists are missing something important about the way Earth's climate system works.
In the early Pliocene Epoch, the world was a much warmer, more temperate planet that it is today. In fact, this brief spurt of warmth we are experiencing today is truly abnormal for the climate of the past 3 million or so years. The CO2 levels of the present day are also significantly higher than the recent average (when I say recent I'm talking in geologic terms; hundreds of thousands of years, not decades or even centuries). Because the Pliocene is the most recent time with consistently warmer climate, scientists have looked to it as a possible analog for a world with higher atmospheric CO2. A group of researchers, led by A. V. Fedorov from Yale University's Department of Geology and Geophysics, recently did a comparison between current conditions and those during the Pliocene. Here is their paper's abstract from Nature:
About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions.
Before examining the differences between climate past and present, it is important to note that, once again, climate models fail to accurately simulate reality. This leads the authors to suggest that various factors are underrepresented in current models. We will return to this point at the end of this article.
At the beginning of the paper, titled “Patterns and mechanisms of early Pliocene warmth,” Fedorov et al. note that the evolution of Earth’s climate over the last five million years (Myr) has been meticulously studied. For many years, the focus of attention was on the origin of the glacial cycles that characterize the current Pleistocene Ice Age. The coming and going of continental ice sheets is documented in the δ18O proxy record reflecting global ice volume and deep-ocean temperature (see the figure below). Three million years ago, the gradual onset and amplification of Northern Hemisphere glaciation was accompanied by a significant drop in atmospheric CO2. This lead many to identify carbon dioxide as the main controller of climate change. Now, a wealth of new data shows that this was only one facet of the changing climate and was caused by the global cooling rather than causing it.
Climate evolution over the past 5 Myr.
The figure above, taken from the article, shows the following: a, Variations in benthic δ18O from deep-ocean sediments, showing glacial cycles (light blue) and a long-term trend (dark blue). δ18O records past variations in deep-ocean temperature and global ice volume. The shading highlights the period during which Northern Hemisphere glaciation gradually begins. b, Magnetic susceptibility of ocean sediments in the northern Pacific (green) and Atlantic (red). The units are nondimensional and the data are presented as originally published. Higher susceptibility indicates the presence of more ice-rafted debris, corresponding to larger Northern Hemisphere ice sheets. ODP is the Ocean Drilling Program. c, Proxy pCO2 data from marine geochemical studies and ice-core inclusions (pCO2, partial pressure of CO2). Uncertainties, indicated by the widths of the data bands, are defined in the respective studies. The two dashed lines indicate the preindustrial (280 ppm) and current (390 ppm in 2010) concentrations of atmospheric CO2.
As the authors note, the differences between the warm Pliocene and the much colder Pleistocene are relatively small. CO2 diminished, but the wholesale changes in climate were far out of proportion to that change. Here is how Fedorov at al. summarized the changes:
The early Pliocene itself, the warm interval that preceded the glaciation, has attracted a lot of attention as a possible analogue for future climate conditions. Despite relatively small differences in climate control factors, including CO2 concentrations (Fig. 1c), between the early Pliocene and the present, the former was markedly different. Palaeorecords indicate vast changes in climate patterns since 5–4 Myr ago, including a contraction of the tropical belt and oceanic warm pool, emergence of strong temperature gradients along the Equator, cooling of coastal upwelling zones in the subtropics, the shoaling of the ocean thermocline, and cooling of the high-latitude and deep ocean. Together these observations imply a large structural change in climate, with major global and regional implications.
The paper goes on to construct detailed histories for sea surface temperature (SST) at various latitudes and locations. This is because, in order to understand climate change, you need to address changes in various places around the planet—average global temperature is fairly meaningless. The figure below highlights this.
Temperatures over the past 5Myr.
In the upper panels, the solid lines are 400-kyr running means for records that resolve Milankovitch (orbital) timescales, whereas dashed lines are 600-kyr running means of time series with a coarser resolution. As several of these records indicate, the period from 4.4 to 4 Myr ago was probably the warmest interval within this time interval—the Pliocene climatic optimum. The subsequent cooling started at roughly the same time in both hemispheres and involved regions ranging from low-latitude upwelling zones to mid and high latitudes. The expansion of the Northern Hemisphere ice sheets around 2.7 Myr ago—evidenced by an increase in the magnetic susceptibility of sediments affected by ice-rafted debris as documented in the first figure—introduced a strong asymmetry in the climate evolution over land in the two hemispheres, but this is less prominent over the ocean.
So there was something else afoot here, aside from an observed dip in CO2 levels. One of the things that happened during the last 5 million years is the closing of the land bridge between North and South America. Originally, the Central American Seaway (CAS) closure was thought to have influenced the onset of glaciation around 2.7 Myr ago, but recent studies indicate that the closure occurred between 4.7 and 4.2 Myr ago. More recently, it has been suggested that the seaway closure could have facilitated subsequent cooling. The authors, however, do not agree, at least based on their model:
Opening the CAS in our model to a depth of 150 m causes a reduction in the strength of the Atlantic meridional overturning circulation, a cooling of the Northern Hemisphere and a warming of the Southern Hemisphere. However, impacts on the equatorial SSTs are minor. The equatorial thermocline deepens a little in the central Pacific but shoals in the very east. Opening the CAS to the largest possible depth (1,100 m) barely changes the zonal SST gradient along the Equator, but amplifies the interhemispheric seesaw pattern with a strong cooling in the northern high latitudes, contradicting the observations. Similar behaviour is seen in other models.
The implication is that the great cooling leading up to the Pleistocene Ice Age cannot be blamed on the formation of the Isthmus of Panama alone. Still, the ever drifting continents are a reminder that Earth itself is constantly changing. During the mid-Pliocene global sea level was 25m higher, there was no significant ice sheet on Greenland and some of the continents may have been up to 250km from their present locations. Our Earth is not the same planet as the Earth of the Pliocene.
Probably the single global climate difference between what the warmist crowd's models are predicting and the climate of the Pliocene is the temperature gradient between the tropics and more temperate latitudes. Contrary to what the climate catastrophists claim, a warmer climate tends to suppress tropical storm activity because circulation becomes more uniform. Even under significant warming the tropics do not heat up, they simply expand. The higher latitudes are where the temperatures climb. This is in agreement with historical records that indicate more major storms during the centuries of the Little Ice Age, 300-400 years ago.
Here is how the paper concludes:
Palaeorecords allow us to identify critical features of the early Pliocene climate: it was a warm climate characterized by a minimal increase in warm pool SST but substantially weaker meridional and zonal SST gradients, and, hence, weaker atmospheric circulation and a deeper tropical thermocline. These differences between the early Pliocene and modern climates amount to structural climate change, regardless of whether we consider trends in mean temperatures or interglacial temperatures (the latter are less affected by continental ice sheets than are the former).
Overall, we are just beginning to understand the driving forces behind climate evolution since the early Pliocene. The climatic changes occurred in concert with a relatively small reduction—less than 50–100 p.p.m.—in atmospheric CO2. Either this reduction pushed the system over a threshold, leading to the structural climate change, or the climate system was cooled by other mechanisms and CO2 provided a positive feedback for the cooling.
The possibility of a tipping point is merely grasping at straws since no mechanism to affect such a change has been identified. Still, being even handed requires not discounting the role of CO2 altogether, though the importance of the posited feedback mechanism must be fairly low since raising the level by 100ppm did not return Earth to a pre-Ice Age climate (CO2 levels have been so elevated during each interglacial period over the past 1.2 million years). My interpretation of these findings is that, when Earth began cooling 4 million years ago, something important in the climate system changed and it was not CO2. CO2 levels simply followed the temperature trend, as they have for the past half a billion years.
Human CO2 emissions may well have a slight warming effect on Earth's climate but there are much larger influences at work here, influences that science is still trying to identify. Unsurprisingly, it is obvious that models based on CO2 as the primary driver of climate change simply cannot reproduce historical conditions—reinforcing the fact that CO2 is not the proximate cause of climate change. When the next glacial period starts the fools who blather on about the “cost of carbon” will find out about the true cost of cold. When that day comes, our descendents will pray for global warming.
Be safe, enjoy the interglacial and stay skeptical.