THIS POST IS A CRITICAL REVIEW OF THE CLAIM BY NASA CLIMATE SCIENTISTS THAT THE CURRENT WARMING CYCLE OF THE HOLOCENE MUST BE HUMAN CAUSED BECAUSE IT CANNOT BE EXPLAINED IN TERMS OF THE MILANKOVITCH THEORY OF GLACIATION CYCLES.

LINK TO SOURCE: https://climate.nasa.gov/blog/2949/why-milankovitch-orbital-cycles-cant-explain-earths-current-warming/

PART-1: WHAT THE SOURCE ARTICLE SAYS

In the last few months, a number of questions have come in asking if NASA has attributed Earth’s recent warming to changes in how Earth moves through space around the Sun: a series of orbital motions known as Milankovitch cycles. What cycles, you ask? Milankovitch cycles include the shape of Earth’s orbit (its eccentricity), the angle that Earth’s axis is tilted with respect to Earth’s orbital plane (its obliquity), and the direction that Earth’s spin axis is pointed (its precession). These cycles affect the amount of sunlight and therefore, energy, that Earth absorbs from the Sun. They provide a strong framework for understanding long-term changes in Earth’s climate, including the beginning and end of Ice Ages throughout Earth’s history. (You can learn more about Milankovitch cycles and the roles they play in Earth’s climate here). But Milankovitch cycles can’t explain all climate change that’s occurred over the past 2.5 million years or so. And more importantly, they cannot account for the current period of rapid warming Earth has experienced since the pre-Industrial period (the period between 1850 and 1900), and particularly since the mid-20th Century. Scientists are confident Earth’s recent warming is primarily due to human activities — specifically, the direct input of carbon dioxide into Earth’s atmosphere from burning fossil fuels.

So how do we know Milankovitch cycles aren’t to blame?

First, Milankovitch cycles operate on long time scales, ranging from tens of thousands to hundreds of thousands of years. In contrast, Earth’s current warming has taken place over time scales of decades to centuries. Over the last 150 years, Milankovitch cycles have not changed the amount of solar energy absorbed by Earth very much. In fact, NASA satellite observations show that over the last 40 years, solar radiation has actually decreased somewhat. Second, Milankovitch cycles are just one factor that may contribute to climate change, both past and present. Even for Ice Age cycles, changes in the extent of ice sheets and atmospheric carbon dioxide have played important roles in driving the degree of temperature fluctuations over the last several million years. The extent of ice sheets, for example, affects how much of the Sun’s incoming energy is reflected back to space, and in turn, Earth’s temperature. Then there’s carbon dioxide. During past glacial cycles, the concentration of carbon dioxide in our atmosphere fluctuated from about 180 parts per million (ppm) to 280 ppm as part of Milankovitch cycle-driven changes to Earth’s climate. These fluctuations provided an important feedback to the total change in Earth’s climate that took place during those cycles. Today, however, it’s the direct input of carbon dioxide into the atmosphere from burning fossil fuels that’s responsible for changing Earth’s atmospheric composition over the last century, rather than climate feedbacks from the ocean or land caused by Milankovitch cycles. Since the beginning of the Industrial Age, the concentration of carbon dioxide in Earth’s atmosphere has increased 47 percent, from about 280 ppm to 412 ppm. In just the past 20 years alone, carbon dioxide is up 11 percent. Scientists know with a high degree of certainty this carbon dioxide is primarily due to human activities because carbon produced by burning fossil fuels leaves a distinct “fingerprint” that instruments can measure. Over this same time period, Earth’s global average temperature has increased by about 1 degree Celsius (1.8 degrees Fahrenheit), and is currently increasing at a rate of 0.2 degrees Celsius (0.36 degrees Fahrenheit) every decade. At that rate, Earth is expected to warm another half a degree Celsius (almost a degree Fahrenheit) as soon as 2030 and very likely by 2040. This relatively rapid warming of our climate due to human activities is happening in addition to the very slow changes to climate caused by Milankovitch cycles. Climate models indicate any forcing of Earth’s climate due to Milankovitch cycles is overwhelmed when human activities cause the concentration of carbon dioxide in Earth’s atmosphere to exceed about 350 ppm. Scientists know of no natural changes to the equilibrium between the amount of solar radiation absorbed by Earth and the amount of energy radiated back to space that can account for such a rapid period of global warming. The amount of incoming solar radiation has increased only slightly over the past century and is therefore not a driver of Earth’s current climate warming. Since 1750, the warming driven by greenhouse gases coming from the human burning of fossil fuels is over 50 times greater than the slight extra warming coming from the Sun itself over that same time interval. If Earth’s current warming was due to the Sun, scientists say we should expect temperatures in both the lower atmosphere (troposphere) and the next layer of the atmosphere, the stratosphere, to warm. Instead, observations from balloons and satellites show Earth’s surface and lower atmosphere have warmed but the stratosphere has cooled. Finally, Earth is currently in an interglacial period (a period of milder climate between Ice Ages). If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago. There’s nothing cool about that.

PART-2: CRITICAL COMMENTARY

THE FIRST ISSUE HERE IS TIME SCALE. THE MILANKOVITCH CYCLE WAS PROPOSED TO EXPLAIN GLACIATION CYCLES AT TIME SCALES OF 100,000 TO 200,000 YEARS. IT IS NOT POSSIBLE TO TEST THAT THEORY AGAINST WARMING AND COOLING CYCLES OF AN INTERGLACIAL PERIOD AT CENTENNIAL AND MILLENNIAL TIME SCALES.

THE SECOND ISSUE IS THE HOLOCENE INTERGLACIAL TEMPERATURE CYCLES EXPLAINED IN A RELATED POST: https://tambonthongchai.com/2021/08/17/the-oddity-of-an-interglacial-warming-cycle/

WHAT WE FIND IN THAT RELATED POST IS THAT THE CURRENT WARMING CYCLE IS NOT A UNIQUE OCCURRENCE OF THE THIS INTERGLACIAL BUT THAT IT IS PART OF A 9,000 YEAR SEQUENCE OF ALTERNATING WARMING AND COOLING CYCLES AT 100 TO 1000 YEAR TIME SCALES. TO USE THE FAILURE OF THE MILANKOVITCH CYCLE TO EXPLAIN THE CURRENT WARMING AS PROOF OF HUMAN CAUSE IT MUST BE SHOWN THAT THE MILANKOVITCH CYCLE EXPLAINS THE OTHER WARMING AND COOLING CYCLES OF THE HOLOCENE AS FOR EXAMPLE THE HOLOCENE CLIMATE OPTIMUM, THE MINOAN WARM PERIOD, THE ROMAN WARM PERIOD, THE MEDIEVAL WARM PERIOD AND THE INTERVENING COOLING PERIODS AS IN THE DEVASTAING 8.2K COOLING. WHAT WE FIND IN THE DATA FOR BOTH THE HOLOCENE AND THE PREVIOUS INTERGLACIAL THE EEMIAN, IS THAT INTERGLACIALS ARE NOT AT A STEADY TEMPERATURE BUT VIOLENT CYCLES OF WARMING AND COOLING AT TIME SCALES OF 100 TO 1000 YEARS. THE UNIQUENESS OF THE CURRENT WARMING CYCLE OF THE HOLOCENE CAN BE ESTABLISHED IN TERMS OF THE FAILURE OF THE MILANKOVITCH THEORY TO EXPLAIN IT ONLY IF THE MILANKOVITCH THEORY CAN BE SHOWN TO EXPLAIN THE EIGHT OTHER TEMPERATURE CYCLES OF THE HOLOCENE.