Climate change is occurring all around us. However, we might not notice its change on a day-to-day basis. In fact, it is an ongoing process of variation that has taken place over millions of years. In order to understand how Earth’s temperature has changed throughout time, researchers today must learn about past climate conditions through the examination of fossils, landmasses, human activity, and more.
Since the demise of dinosaurs 65 million years ago (mya), overall global temperature has averaged as high as 6° Celsius above (Eocene 50–55 mya) to as low as 6° Celsius below (during the Pleistocene less than 2 mya) our present-day climate. During the last interglacial warm period, known as the Eemian, temperatures averaged around 2° Celsius above our current levels. Later, just 20,000 years ago, temperatures plummeted to 6° Celsius below current values during the last glacial maximum (LGM). The most recent period of temperatures to be above present-day levels occurred during the mid-Holocene era, around 6,000 years ago, when minor oscillations in Earth’s orbit sparked temperatures that averaged 1° Celsius higher than the current average (at least in the Northern Hemisphere).
Although there are significant uncertainties determining how much warming can be expected during the remainder of the twenty-first century, the present rate of Earth’s radiative imbalance (0.37 W/m2)1 would result in an average warming of 2° Celsius. Predicting these changes will require improvement in our understanding of water vapor feedbacks, precipitation, cloud behavior, and other natural and human-induced effects to significantly refine the accuracy of any climate projections. However, if a heating rate of 0.37 W/m2 were to continue, Earth’s overall temperature would be close to the peak of the Eemian interglacial (125,000 years ago) by the end of this century. During the Eemian, hippopotamus and other tropical animal fossils were found in England, which had summers up to 4° Celsius warmer than today.2 These fauna illustrate the effect that a global 2° Celsius warming could potentially have in certain regions.
Ocean currents also affect the distribution of heat in a warming or cooling climate—and continental configurations significantly impact ocean circulation. The separation of Antarctica from both South America and Australia3 more than 35 mya allowed for the formation of an ocean water “pump,” which resulted in considerable global cooling. Additionally, the rise of the Himalayan Mountains since that time has added to the decrease in global temperature. The presence of the Himalayas has likely caused increases in planetary albedo (reflectivity) and changes to the atmospheric circulation of the Northern Hemisphere. More recently, between 3 to 5 mya, during the Pliocene epoch, the development of the Isthmus of Panama and the collision of Africa and Europe formed the Mediterranean Sea. Both of these events, especially the formation of Panama, presumably had a role in Earth’s further cooling from the resulting changes in ocean currents.
Since the Pliocene, the Atlantic and Pacific waters have not been able to exchange heat between the Americas. In fact, more of the Atlantic’s warm but salty tropical waters were directed toward the Northern Atlantic. Warmer waters in the North Atlantic tended to enhance evaporation, which may have allowed for increased snowfall in both the Arctic and Europe (again increasing albedo). The mid-Pliocene era (3 mya) holds the record for the warmest temperatures to occur within the timeframe of today’s configuration of continents and associated landmasses. At times, the mid-Pliocene temperatures were about 3° Celsius above present values.
The variation in temperature we may see over the next century will be determined by a host of human and natural factors, both having warming and cooling tendencies. Some human influences include greenhouse gas levels, aerosol type and distribution, land cover alteration, and changes in energy use. Natural factors might include oceanic temperature oscillations, solar activity changes, and volcanism. If a warming of 3° Celsius represents maximum natural global temperature conditions given the current land and ocean configurations, this value could represent a reasonable starting point in attempts to understand limits to both man-made and natural feedback in the climate system.
Regardless, it is amazing how well the Earth has maintained a moderate temperature range over a vast period of time. This temperature balance directs us back to the Creator, who well-crafted the continental and oceanic configurations that continue to impact our climate today (Proverbs 8:29–30).
Dr. Kevin Birdwell
Kevin R. Birdwell received his PhD from the University of Tennessee in 2011 and currently serves as a meteorologist and atmospheric researcher in Oak Ridge, Tennessee.