Over the past century, the earth’s average temperature has increased rapidly increased 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 the thousands of years before the industrial revolution, before thermometers and before humans warmed the climate? Release of heat-binding 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 have emerged 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, 2023propose 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 climate, or paleoclimatelook for temperature data from way back in time, long before thermometers and satellites.
We have two choices: We can find stored information about past climates in natural archivesor we can simulate the past with climate models.
There are several nature archives that record changes in climate over time. The growth rings that form in every year trees, stalagmites, And corals can be used to reconstruct past temperatures.
Similar data can be found in glacial ice and found in tiny shells in the Sediment deposited on the seabed over time 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 ocean-dwelling creatures foraminifera. If a foraminifera is alive, the chemical composition is theirs 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 sea floor. 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 it study current conditions, forecast changes in the future, And reconstruct the past.
For example, scientists can input past greenhouse gas concentrations that we know Information stored in tiny bubbles in ancient ice, and the model can use this 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 overview paperwe assessed 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 variability Earth’s orbit around the sungreenhouse gas concentrations, Volcanic eruptions, And the strength of the thermal energy of the sun.
We also studied important climatic feedbacks such as vegetation and sea ice changes that can do this affect global temperature.
For example, there is strong evidence for this less arctic sea ice And more vegetation cover existed about 6,000 years ago than 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 records generally show that the Earth’s average temperature was about 6,000 years warmer about 0.7 °C (1.3 °F) compared to the 19th-century median, 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 Agricultural societies developed in the millennia thereafter Ice Shields retreated on the northern hemisphere.
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.
One climate model experiment which 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.
Understand and better incorporate this and other feedback will be important while scientists improve our ability to predict future changes.
Ellie BroadmanPostdoc in climate science, University of Arizona And Darrell KaufmanProfessor of Earth and Environmental Sciences, University of Northern Arizona
This article is republished by The conversation under a Creative Commons license. read this original article.
