Was the Earth warming or cooling before global warming? Study says…

Was Earth already warming, or did global warming reverse a long-term cooling trend?

Arizona:

Over the past century, Earth’s average temperature has rapidly increased by about 1 °C (1.8 °F). It is difficult to dispute the evidence. 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?

At that time, was the Earth’s temperature warming or cooling?

Even though scientists know more about the most recent 6,000 years than about any other multimillennial interval, studies on this long-term global temperature trend have come to opposite conclusions.

To try to resolve the gap, we conducted a comprehensive, global-scale assessment of the existing evidence, including both natural archives, such as tree rings and seafloor sediments, and climate models. Our results, published February 15, 2023, suggest ways to improve climate forecasting to avoid missing some important slow-moving, naturally occurring climate feedbacks.

in the context of global warming

Scientists like us who study the climate of the past, or palaeoclimate, look for temperature data from thermometers and satellites from much earlier times.

We have two options: we can obtain information about past climate stored in natural archives, or we can simulate the past using climate models.

There are many natural records that record changes in climate over time. Growth rings that form each year in trees, stalagmites and corals can be used to reconstruct past temperatures. Similar data can be found in small shells found in glacier ice and in sediments that have formed over time at the bottom of oceans or lakes. These act as substitutes or proxies for thermometer-based measurements.

For example, changes in the width of tree rings can record fluctuations in temperature. If temperatures are too cold during the growing season, the tree ring formed that year is thinner than in a year with warmer temperatures.

Another temperature proxy is found in sea floor sediments, in the remains of tiny ocean-dwelling organisms called foraminifera. When a foraminifer is alive, the chemical composition of its shell changes depending on the temperature of the ocean. When it dies, the shell sinks and over time is buried by other debris, forming layers of sediment on the ocean floor. Paleoclimatologists can then extract sediment cores and chemically analyze shells in those layers to determine their composition and age, sometimes for millennia.

Climate models, our other tool for exploring past climates, are mathematical representations of Earth’s climate system. They model the relationships between the atmosphere, biosphere and hydrosphere to create our best replica of reality.

Climate models are used to study current conditions, predict future changes, and reconstruct the past. For example, scientists can input past concentrations of greenhouse gases, which we know from information stored in tiny bubbles in ancient ice, and the model can use that information to simulate past temperatures. Modern climate data and details from natural archives are used to test their accuracy.

Proxy data and climate models have different strengths.

The proxies are tangible and measurable, and they often have well-understood responses to temperature. However, they are not evenly distributed around the world or over time. This makes it difficult to reconstruct global, continuous temperatures.

In contrast, climate models are continuous in space and time, but while they are often very efficient, they will never be able to capture every detail of the climate system.

a paleo-temperature puzzle

In our new review paper, we assess climate theory, proxy data and model simulations, focusing on indicators of global temperature. We carefully considered naturally occurring processes that affect climate, including long-term changes in Earth’s orbit around the Sun, greenhouse gas concentrations, volcanic eruptions and the strength of the Sun’s heat energy.

We also examined important climate feedbacks, such as vegetation and sea ice changes, that may affect global temperatures. For example, there is strong evidence that there was less Arctic sea ice and greater vegetation cover during the period approximately 6,000 years ago than during the 19th century. This would have darkened the Earth’s surface, allowing it to absorb more heat.

Our two types of evidence give different answers about the trend of Earth’s temperature over the 6,000 years before modern global warming. Natural archives generally show that Earth’s average temperature around 6,000 years ago was about 0.7 C (1.3 F) warmer than in the mid-19th century, 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 modest warming trend consistent with a gradual increase in carbon dioxide as agriculturally based societies developed over the millennia after the ice sheets retreated in the Northern Hemisphere.

How to improve climate forecasts

Our assessment highlights some ways to improve climate forecasts.

For example, we found that models would be more powerful if they more fully represented certain climate responses. A climate model experiment that included increased vegetation cover in some regions 6,000 years ago Unlike most other model simulations, which do not include this expanded vegetation, we were able to simulate the global temperature peak seen in the proxy record Were.

Understanding and better incorporating these and other feedbacks will be important as scientists continue to improve our ability to predict future changes.

,Author:Ellie Brodman, Postdoctoral Research Associate in Climate Science, University of Arizona and Darrell Kaufman, Professor of Earth and Environmental Sciences, Northern Arizona University)

,disclosure statement: Ellie Brodman has received funding from the National Science Foundation, the University of Arizona, and Northern Arizona University. Darrell Kaufman receives funding from the National Science Foundation)

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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