The Fall of Mann

Over 300 years ago, humanity suffered greatly in the depths of the Little Ice Age. Thankfully, cold temperatures reached their nadir in the late 1600’s and began to rise thereafter. After around a century of warming, the glaciers ceased to advance and instead began to slowly recede. Water from the melting glaciers found its way to the oceans and sea levels began to rise. All this took place long, long before there was any rise in carbon dioxide levels due to human activity.

Prior to the Little Ice Age, the earth enjoyed a period of warmer temperatures known as the Medieval Warm Period. The Medieval Warm Period lasted from roughly 900 until 1300. During this time period, temperatures were about 2-4 F degrees warmer than today. Tellingly, periods of warmer temperatures are often referred to as “climate optima” because they are so favorable to humans and other forms of life. Indeed, the Medieval Warm Period was a time of human flourishing. Vikings used this time to colonize southern Greenland. For several hundred years, the Greenland Vikings built homes and churches, planted crops and raised livestock. But temperatures began to fall around 1300 and eventually these settlements had to be abandoned. Also during the Medieval Warm Period, vineyards were planted in England, far north of where grapes can be grown today. Warmer temperatures and longer growing seasons enabled crops to be raised in previously inhospitable regions of Scandinavia.

The graph below – clearly showing the Medieval Warm Period and the subsequent Little Ice Age – was included in the First Assessment Report issued in 1990 by the United Nation’s Intergovernmental Panel of Climate Change (IPCC).

Sadly, many climate alarmists have attempted to undermine this well-established understanding of the earth’s climate over the last millennium. Foremost among these alarmists is Michael Mann. In 1998, Mann and two colleagues published a paper claiming that temperatures had minimally cooled between 1000 and 1900 AD, then suddenly spiked upward in the last one hundred years. This paper represents the birth of the infamous “hockey stick.” Mann’s temperature reconstruction earned this moniker because the temperature plot resembled a hockey stick. The long, slow decline in temperature was the shaft of the stick while the dramatic upswing in recent temperature was the blade. It would be difficult to overstate the damage this proposal has had on climate science. Mann’s single paper completely eliminated the Medieval Warm Period and the Little Ice Age, despite mountains of evidence documenting the reality of these time periods. The hockey stick became the crown jewel of the IPCC’s Third Assessment Report in 2001. It was also featured in Al Gore’s climate alarmist movie “An Inconvenient Truth.” This dramatic revision of the earth’s climate history has been uncritically accepted by many in the media.

Given its enormous impact, Michael Mann’s paper deserves careful scrutiny. What do we find when we look behind the curtain? How did Mann discover 900 years of gradual cooling temperatures rather than the well-documented Medieval Warm Period followed by the Little Ice Age? It turns out that Mann’s paper is deeply and fatally flawed. Mann’s temperature reconstruction relies heavily on tree ring data mainly from bristlecone pines in California. Most people are aware that scientists can learn about a tree’s history by looking at its growth rings. Wide growth rings indicate excellent growing conditions while narrow growth rings indicate difficult growing conditions. Trees grow more readily when there are warmer temperatures, abundant rainfall, plenty of soil nutrients and lack of pests. Now scientists are keenly aware that tree rings are not a reliable way to determine past temperature because their growth depends on multiple factors – not just temperature. Warmer temperatures and longer growing seasons do help trees grow faster and produce wider growth rings. However, rainfall plays the largest role in tree growth. Therefore, trees will grow quickly during a year of moderate or even cool temperatures as long as they have plenty of rainfall.

Despite the uncertain nature of tree rings, Mann used them anyway. Moreover, he didn’t use all the tree ring data available, but selected only the data which would produce the hockey stick. Such “cherry-picking” of data is a dreadful violation of scientific integrity. Just as disturbingly, Mann also used complex mathematical trickery to manipulate the data. When other researchers requested that Mann show his methods so that they could double check his work, Mann steadfastly refused to do so. The sad fact is that Mann’s paper is a disaster. Many scientists have challenged the faulty nature of Mann’s work, but in the meantime climate alarmists have used the hockey stick to their full advantage. Appallingly, alarmists in the media and the political world continue to use Mann’s temperature reconstruction to claim that current temperatures are by far the warmest in the last 1000 years – despite clear evidence that temperatures were warmer during the Medieval Warm Period.

There are many scientists who acknowledge that Mann’s hockey stick has been thoroughly discredited. Others go further, calling Mann’s work intentionally fraudulent. Canadian climatologist Dr. Tim Ball once quipped that Michael Mann belongs “in the State Pen, not Penn State,” implying that Mann deserves a jail cell rather than a prestigious academic post. This situation escalated when Mann brought a defamation lawsuit against Ball in the Canadian court system. The court dismissed Mann’s lawsuit in August 2019 when he again refused to submit his data for review. At the end of the day, Michael Mann and his hockey stick should serve as a stark warning. Science can be a powerful tool to help us understand our world. But when science loses its moorings and ceases to honestly and dispassionately pursue the truth, it can do tremendous damage. Sadly, it may take climate science many years to recover from this sordid affair.

So what can sound science teach us about the earth’s temperature over time? Well, as we’ve noted, worldwide temperatures have been increasing since the depths of the Little Ice Age in the late 1600’s. This 300 year warming trend began long before human CO2 emissions could have had any possible impact on climate. Moreover, there remains strong evidence that temperatures from the Medieval Warm Period significantly exceeded today’s temperatures. In other words, our modern temperatures are far from unprecedented, even in the last 1000 years!

Are we able to determine temperatures even further back in time? It turns out that scientists do have solid methods for assessing ancient temperatures. You will recall the Antarctic ice cores which were used to determine CO2 levels in the past by sampling the air bubbles trapped in the ice. Reconstructions of ancient temperatures can also be derived from these ice cores by measuring the ratio of hydrogen or oxygen isotopes. The Antarctic ice cores allow us to look back 800,000 years ago. This record shows cycles of colder and warmer temperatures. The earth has experienced long ice ages roughly 100,000 years in duration. These ice ages have been interrupted by shorter periods of interglacial warming lasting about 10 to 15 thousand years. Thankfully, we are currently enjoying one of these warm interglacial periods which has allowed life on earth to flourish – including humanity.

Reviewing the temperature record, we can discern that each of the last four interglacial warm periods have produced higher temperatures versus the present. Again, these warmer temperatures obviously have nothing to do with any human activity. The most recent interglacial warm period is known as the Eemian. Temperatures during the Eemian peaked about 120 thousand years ago and were substantially higher than modern day temperatures. This analysis begs the question: why have temperatures been cycling up and down in this manner? One very plausible answer relates to astronomical changes. It turns out that the earth’s orbit is not quite a perfect circle as most people imagine. The earth’s orbit around the sun is actually slightly eccentric – in other words, our path around the sun is a subtle ellipse. And the shape of this ellipse varies in 100,000 year cycles. Additionally, there are changes related to the earth’s axis. The tilt of the earth’s axis varies from 22.1 degrees to 24.5 degrees in a 41,000 cycle. The earth’s axis also precesses or wobbles very slightly like a top. The cycle of each wobble lasts about 26,000 years. These three cycles are collectively known as the Milankovitch Cycles and are believed to affect the amount of solar energy reaching the earth, thereby influencing the climate.

Even if we focus in on the current interglacial temperature record, we see that temperatures have often been warmer in the past. The graph below shows the temperature over the last 10,000 years. (Note that the data ends in 1855, just after the Little Ice Age. Since that time temperatures have blessedly warmed, but are still well below the Medieval Warm Period.) From this graph we can see that temperatures over the last 10,000 years have been warmer than modern temperatures more than 60% of the time! Moreover, the earth’s past temperatures have changed rather quickly (in geologic terms), so that today’s warming temperatures are occurring at rates very similar to those in the past.

Let’s conclude our examination of the earth’s temperature by stepping back and looking at the really big picture. The graph below shows a global temperature reconstruction over many millions of years. We can see that the earth’s temperatures have alternated between warmer “hothouse” conditions and colder “icehouse” conditions. The fact is that the current geologic period- the Quaternary – is among the coldest times in the earth’s long history! The earth has not experienced temperatures this cold since the early Permian period more than 250 million years ago. For most of the last 600 million years, “hothouse” conditions have prevailed. During these “hothouse” times, temperatures have often averaged 18 F (10 C) degrees warmer than today! Additionally, most people would be shocked to learn that for much of the earth’s history there has been little or no ice at the earth’s poles. In fact, it was not until around 3 million years ago that ice returned to the North Pole!

So what can we conclude from our analysis of the earth’s temperatures? The key take home message is that today’s temperatures are by no means unprecedented or unusual. This statement holds true no matter what time scale we use. First, temperatures during the Medieval Warm Period (900 to 1300 AD) exceeded our modern times, despite Michael Mann’s attempts to claim otherwise. Second, temperatures over the last 10,000 years, during the current interglacial warm period, have often exceeded modern temperatures. Third, multiple previous interglacial warm periods – including the most recent Eemian about 120,000 year ago – exceeded modern warmth. And finally, when we look back on the earth’s long history we learn that our modern temperatures are in fact relatively cold. Arctic and Antarctic Ice has been the exception rather than the rule. So take heart! The earth is doing just fine and will happily benefit from a little more warmth.