Over the past century, the Earth’s average temperature has risen rapidly by about 1 degree Celsius (1.8 degrees Fahrenheit).
It’s hard to challenge the evidence. They come 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 by releasing heat-trapping carbon dioxide from fossil fuels?
Did the Earth’s temperature rise or fall during this time?
Although scientists know more about the last 6,000 years than any other multi-millennial time period, studies of this long-term trend in global temperature have come to conflicting conclusions.
To try to address this discrepancy, we conducted a comprehensive global assessment of existing data, including natural archives such as tree rings and seafloor sediments, and climate models.
Findings by Eli Brodman, PhD researcher in climate science, and Darrell Kaufman, professor of earth and environmental sciences at Northern Arizona University, published Feb. 15, 2023, suggest ways to improve climate prediction so you don’t miss out on some. important slow climate responses that say normal.
Global warming in context
Scientists who study paleoclimate, or paleoclimate, have been looking for temperature data for a long, long time, long before thermometers and satellites existed.
We have two options: we can find information about the past climate stored in natural archives, or we can simulate the past using climate models.
There are many natural archives that record climate change over time. The annual rings that form annually on trees, stalagmites, and corals can be used to restore previous temperatures.
Similar data can be found in eternal ice and in small shells found in sediments that accumulate over time on the ocean floor or in lakes. This is an alternative to thermometer-based measurements.
For example, changing the width of annual rings can register temperature fluctuations. If the weather during the growing season was very cold, the year formed that year would be thinner than a year with warmer weather.
Another heat factor has been found in seabed sediments, in the remains of tiny ocean-dwelling creatures called foraminifera. When a foraminifera is alive, the chemical composition of its shell changes depending on the ambient temperature.
As they are used up, they sink and are buried under other debris over time, 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 going back thousands of years. Climate models, another tool for studying past environments, are mathematical representations of the Earth’s climate system. The relationship between the atmosphere, biosphere and hydrosphere is formulated in such a way as to create the best copy 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 concentrations of greenhouse gases that we know from information stored in tiny bubbles in ancient ice, and a model can use that information to model past temperatures. The latest climate data and details from nature archives are used to verify their accuracy.
Climate models are continuous in space and time, but although they are often very clever, they never capture every detail of the climate system.
ancient temperature mystery
In a new review paper, researchers evaluate climate theory, surrogate data and modeling with a focus on global temperature indices.
They carefully studied the natural processes that affect climate, including long-term changes in the Earth’s orbit around the Sun, greenhouse gas concentrations, volcanic eruptions and the strength of solar thermal energy.
They also studied important climate feedbacks, such as changes in land cover and sea ice, that could affect global temperatures.
For example, there is strong evidence that the Arctic had less sea ice and more vegetation around 6,000 years ago than it did in the 19th century. This could darken the Earth’s surface, causing it to absorb more heat.
The two types of data give different answers regarding the trend in the Earth’s temperature 6,000 years before today’s global warming.
Natural archives generally show that the Earth’s average temperature nearly 6,000 years ago was about 0.7 degrees Celsius (1.3 degrees Fahrenheit) warmer than the 19th century average, and then gradually declined until the Industrial Revolution.
Meanwhile, climate models generally show a slight warming trend consistent with a gradual increase in carbon dioxide as farming societies develop over the millennia after the northern hemisphere ice sheets retreated.
How to improve climate forecasts
The researchers’ assessment points to some ways to improve climate predictions.
For example, they found that models would be more reliable if they fully represented specific climate responses.
One climate model experiment that included an increase in vegetation in some areas 6,000 years ago was able to simulate the global temperature peak seen in surrogate records, unlike most other simulations that do not include this extended vegetation.
A better understanding and integration of these and other observations will be essential as scientists continue to improve our ability to predict future changes.
Source: Science Alert