Over the past century, the Earth’s average temperature has risen rapidly by about 1 degree Celsius (1.8 degrees Fahrenheit). The evidence is difficult to dispute. It comes from thermometers and other sensors around the world.
But what about thousands of years before the Industrial Revolution, before thermometers, and before humans warmed the climate by releasing heat-trapping carbon dioxide from fossil fuels?
Did the temperature of the earth warm up or cool down at that time?
Although scientists know more about the past 6,000 years than any other multi-millennial interval, studies of this long-term global temperature trend come to opposite conclusions.
To try to resolve the difference, we conducted a comprehensive, global review of the existing evidence, including both natural archives, such as tree rings and seafloor sediments, and climate models.
Our results, published on February 15, 2023, suggest ways to improve climate prediction to avoid overlooking some important, slow-moving, naturally occurring climate feedbacks.
Global warming in context
Scientists like us who study past climates, or paleoclimates, are looking for temperature data from way back in time, long before thermometers and satellites.
We have two choices: we can find information about the past climate stored in natural archives, or we can simulate the past using climate models.
There are several nature archives that record changes in climate over time. The annual rings that form in trees, stalagmites and coral each year can be used to reconstruct past temperatures.
Similar data is found in glacial ice and in tiny shells in the sediment that accumulates over time on the bottom of oceans or lakes. These serve as a replacement or proxy for thermometer-based measurements.
For example, changes in the width of tree rings can occur record temperature changes. If the temperature is too cold during the growing season, the annual ring that forms that year will be thinner than it would be in a warmer temperature year.
Another temperature proxy is found in seafloor sediment, in the remains of tiny marine life called foraminifera. When a foraminifera is alive, the chemical composition of its shell changes depending on the temperature of the ocean.
As it dies, the shell sinks and is buried by other debris over time, creating layers of sediment on the seafloor. Paleoclimatologists can then extract sediment cores and chemically analyze the shells in these layers to determine their composition and age, which sometimes goes back thousands of years.
Climate models, our other tool for studying past environments, are mathematical representations of the Earth’s climate system. They model relationships between the atmosphere, biosphere, and hydrosphere to create our best recreation of reality.
Climate models are used to study current conditions, predict changes in the future, and reconstruct the past.
For example, scientists can input past greenhouse gas concentrations, which we know from information stored in tiny bubbles in old ice, and the model can use that information to simulate past temperature. Modern climate data and details from natural archives are used to test their accuracy.
Proxy data and climate models have different strengths.
Proxies are tangible and measurable, and they often have a well-understood response to temperature. However, they are not evenly distributed across the world or over time. This makes it difficult to reconstruct global continuous temperatures.
In contrast, climate models are spatially and temporally continuous, but while they are often very adept, they will never capture every detail of the climate system.
A paleotemperature conundrum
In our new review article, we have evaluated climate theory, proxy data and model simulations, focusing on indicators of global temperature.
We have carefully considered naturally occurring processes that affect climate, including long-term variations in the Earth’s orbit around the Sun, greenhouse gas concentrations, volcanic eruptions, and the strength of the Sun’s thermal energy.
We also examined important climatic feedbacks such as vegetation and sea ice changes that can affect global temperature.
For example, there is strong evidence that about 6,000 years ago there was less Arctic sea ice and more vegetation cover than there was in the 19th century. That would have darkened the earth’s surface and absorbed more heat.
Our two sets of evidence offer different answers regarding Earth’s temperature trends over the 6,000 years before modern global warming.
Natural archives generally show that the Earth’s average temperature was warmer by about 0.7 °C (1.3 °F) about 6,000 years ago compared to the 19th-century mean, and then gradually cooled until the Industrial Revolution. We found that most of the evidence points to this result.
Meanwhile, climate models generally show a slight warming trend, consistent with a gradual increase in carbon dioxide, as agricultural societies developed over the millennia after the northern hemisphere ice sheets retreated.
How to improve climate forecasts
Our assessment highlights some opportunities to improve climate projections.
For example, we found that models would perform better if they more fully represented certain climate feedbacks.
A climate model experiment that included increased vegetation cover in some regions 6,000 years ago was able to simulate the global temperature spike we see in proxy records, unlike most other model simulations, which do not include this extensive vegetation.
Understanding and better incorporating this and other feedback will be important as scientists continue to improve our ability to predict future changes.
Ellie Broadman, Postdoctoral Research Associate in Climate Science, University of Arizona and Darrell Kaufman, Professor of Earth and Environmental Sciences, University of Northern Arizona
This article was republished by The Conversation under a Creative Commons license. Read the original article.