The Intergovernmental Panel on Climate Change (IPCC) has released its latest report, unequivocally stating that human influence has warmed earth’s climate at a rate that is unprecedented in at least the last 2,000 years. The IPCC Report includes a section on the Physical Science Basis that explains the dire consequences that are in store for the earth and humanity due to climate change. The perspective presented is based on the acceptance of a very high degree of certainty that warming during the industrial era is almost entirely a result of human activity, particularly emissions of greenhouse gases, mainly carbon dioxide from combustion of fossil fuels. The narratives presented by the IPCC are fundamentally reliant on the premise that the scenarios represented in Figure SPM.1: History of global temperature change and causes of recent warming should be accepted without question (reproduced as Figure 1 in this post, further down). In this post I examine scientific evidence that should lead us to cast reasonable doubt this degree of certainty is justified.
The public should be made more fully aware that examination of the scientific literature does not support statements suggesting that the “science is settled” and that the viewpoints presented in the IPCC Report are not unanimously endorsed by the scientific community. The reality is that the earth’s climate system is highly complex and many processes and phenomena are still not well understood. This inevitably leads to a situation where mathematical models assumed to be based on a representation of the physical processes should be treated with much caution.
There is an unfortunate tendency, often perpetuated by the media, to automatically assume that the concept of “climate change” must automatically and exclusively be associated with human activities. Overwhelmingly the scientific evidence shows that climate change is part of the natural processes that have continued for thousands and millions of years on planet earth. The relative contribution of natural and human-caused remains poorly understood and it is misleading for the IPCC to suggest otherwise.
The Hockey Stick Reappears
The IPCC figure SPM1a (on the left hand side) represents a reconstruction of global atmospheric temperatures over the past 2020 years. It includes temperatures derived from proxy temperature studies up to 1850 AD.
There are hundreds of studies of proxy temperatures reported in the scientific literature using measurements of characteristics of natural phenomena such as tree rings, corals, ice cores and lake sediments from many different parts of the world. Various techniques have been applied to construct composite profiles (known as temperature reconstructions) of how temperatures have varied over long periods that may extend back over thousands of years and cover large areas of the globe, including regional, hemispheres and global areas. The reconstruction depicted in SPM1a is known as the “hockey stick”, initially introduced by Michael Mann about 20 years ago (due to its apparently similarity to an Ice Hockey stick).
This is characterised by a slow gradual decline in atmospheric temperatures during most of the past 2,000 years, followed by an abrupt temperature increase from about 1850 AD onwards, during the period when instrumental temperature records became available. In this representation of temperatures, there is no distinct Medieval Warm Period (MWP) and current temperatures are unprecedented during the past 2,000 years. The validity of the hockey stick graph has been intensely debated in the scientific literature over the past two decades. This debate includes the validity of methodologies used for construction of multi-proxy temperature, the occurrence of the MWP and Little Ice Age (LIA), whether present temperatures are indeed unprecedented in the past 1,000 years and also the causation of current warming trends.
An alternative representation of temperatures over the past 2,000 years, commonly found in the scientific literature, is illustrated here using an example from a study by Ljungqvist, published in 2010, showing a reconstruction of northern hemisphere temperatures. This temperature reconstruction clearly shows evidence for natural oscillatory processes and major features at maxima corresponding to the Medieval Warm Period (MWP), the Roman Warm Period (RWP) and minima corresponding to the Little Ice Age (LIA) and the Dark Ages (DA). Current temperatures are not unprecedented during the past 2,000 years and are comparable to those experienced during the Medieval Warm Period. We can refer to this type of representation of past temperatures as the oscillatory MWP-LIA profile.
Part of the controversy apparent in the scientific literature has focused on whether there are clearly identifiable features such as the LIA and the MWP in climate records, how extensively these were experienced across the globe, and whether current temperatures are comparable to those in MWP. A review by Soon and Baliunas, published in 2003, considered proxy climatic and environmental changes of the past 1000 years and examined more than 120 examples published between 1975 and 2002. These included regional and worldwide proxy records for both northern and southern hemispheres. A total of 107 studies indicated there was a discernible climatic maximum during the MWP (800 AD–1300 AD) in the proxy record, whereas only six indicated this was absent. There were 123 studies indicating an objectively discernible climatic minimum during the LIA interval (1300 AD–1900 AD) in the proxy record, with only one reporting this was absent.
The IPCC figure SPM1b (on the right hand side of Figure 1, above) shows lines representing the observed global temperature change during the past 170 years and the contribution attributed to natural factors such as solar and volcanic influences. According to this representation from the IPCC Report, natural influences are approximately constant over the past 170 years, and the rate of change contributing to the observed temperature changes over this period is close to zero – the blue line in the figure is essentially horizontal.
If we consider the temperature reconstruction published by Ljungqvist shown above, it essentially represents the natural variation in temperatures over nearly 2,000 years up to 1850AD, the period before there were significant human influences due to combustion of fossil fuels. We see that natural temperature variations of about 0.8°C between the extremes of the MWP and the LIA. The average rate of temperature change between the DA minimum and MWP maximum corresponds to an increase of about 0.15 °C per century. The average rate of temperature change between the MWP maximum and LIA minimum corresponds to a decrease of about 0.15 °C per century. The rate of change between the LIA minimum and 2000AD is about 0.23°C per century, but there is no indication of an abrupt rapid increase during the past century. There may be some influence of human activities on temperature change during the industrial era, but much less exaggerated than suggested by the IPCC.
Proponents of the hockey stick reconstruction, including Michael Mann, have been quoted as refuting the importance of natural oscillatory processes in past temperature records, and asserting that the their consideration is not useful in future projections of temperature or in being able to separate contributions from natural and human influences. This conclusion is not supported by our own published investigations using artificial intelligence, and taking account of the various modes of oscillatory processes revealed in proxy records. These investigations shown that natural processes can probably account for a significant proportion of warming seen in recent decades. They also show that the influence of increasing the concentration of carbon dioxide in the atmosphere (known as the equilibrium climate sensitivity) is probably only about 20% of the value currently accepted by the IPCC.
My article published in the journal Earth Sciences reviewing oscillatory processes in climate change can be freely downloaded here.
There are numerous examples of studies in the scientific literature considering both atmospheric and oceanic temperatures from many parts of the world over different time frames from the past. One example for sea surface temperatures (SSTs) is shown below from a study by Saenger et al., where the temperatures were reconstructed using measurements on corals extending over a period of about 450 years. The temperature profile shows a general decline in temperature from 1550 AD to about 1720 AD, followed by a steady increase moving out from the LIA towards the current period. There is no evidence in this study of a sudden abrupt temperature rise during the past century. This type of temperature profile is very commonly encountered in studies the scientific literature, and is consistent with oscillatory temperature variations with a range of periodicities. The temperature profile is constructed entirely using proxy temperature measurements using coral samples, and does not include instrumental temperature measurements in recent decades.
It has been suggested that one reason for the apparent very abrupt unprecedented rise in temperatures illustrated in Figure 1 showing the hockey stick may be associated with attaching together temperature records derived from very different methodologies without adequate consideration of their quantitative compatibility.
More on the use of proxy data – including from coral cores – can be found in my recent Research Report for the IPA, What Corals Can Teach Us About Climate Change.
Ljungqvist, F. C., A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia. Geografiska Annaler: Physical Geography 2010, 92 A (3), 339-351.
Soon, W., Baliunas, S.,. Proxy climatic and environmental changes of the past 1000 years, Climate Research 2003, 23, 89–110.
Abbot, J., Using Oscillatory Processes in Northern Hemisphere Proxy Temperature Records to Forecast Industrial-era Temperatures. Earth Sciences 2021; 10(3): 95-117
Saenger, C., Cohen , A.L., Oppo , D.W., Halley, R B., Carilli, J.E. Surface-temperature trends and variability in the low-latitude North Atlantic since 1552. Nature Geoscience 2, 492-495, (2009).