Breezy Explainer: Was Earth heating up or cooling down before global warming?

climate

The Earth’s average temperature has rapidly grown by around 1 degree Celsius during the previous century (1.8 degrees Fahrenheit). Hard to refute the proof. It originates from sensors and thermometers all across the planet. But what about the ages before the Industrial Revolution, the invention of thermometers, and the warming of the climate caused by the emission of carbon dioxide from fossil fuels? Was the Earth’s temperature then rising or falling?

Even though research on this long-term global temperature pattern has produced conflicting results, scientists still know more about the past 6,000 years than they do about any other multimillennial period. We assessed all available information, including climate models and natural archives like tree rings and seafloor sediments, on a large scale to try to reconcile the discrepancy. Our findings, which were released on February 15, 2023, include recommendations for how to enhance climate forecasting in order to prevent missing some significant, slowly moving natural climate feedback.

There are numerous natural repositories that document how the climate has changed over time

Researchers like us who study the past climate, or paleoclimate, search for historical temperature records from a long time ago when thermometers and satellites were available. There are two possibilities available to us: either we can use climate models to replicate the past or we can find information about previous climate preserved in natural archives.

There are numerous natural repositories that document how the climate has changed over time. It is possible to reconstruct historical temperatures using the growth rings that corals, stalagmites, and trees form each year. Similar information can be discovered in glacier ice and in the small shells that are embedded in the silt that accumulates over time at the bottom of lakes and the ocean. These act as stand-ins, or proxies, for measurements made with thermometers.

The tree ring that year will be narrower than one from a year with milder temperatures if the growing season is too cold

Trees are the best-known natural archives. Here are several others that hold evidence of past temperature. Cores or other samples from these archives can be used to reconstruct changes over time. Viktor O. Leshyk, Author provided. For instance, variations in the breadth of tree rings might capture temperature changes. The tree ring that year will be narrower than one from a year with milder temperatures if the growing season is too cold.

Sediment from the seafloor contains the remains of small foraminiferal organisms, which are a different type of temperature proxy. When a foraminifer is living, the chemical makeup of its shell varies according to the ocean’s temperature. When an animal dies, its shell sinks and eventually becomes buried under other detritus, creating layers of sediment on the ocean floor. Then, using sediment cores, paleoclimatologists may identify the composition and age of the shells in those layers, sometimes dating back thousands of years.

An article author, Ellie Broadman, to the right, is holding a sediment core taken from a lake on Alaska’s Kenai Peninsula. Our second tool for investigating historical environments is climate models, which are mathematical depictions of the Earth’s climate system. In order to build our best representation of reality, they model interactions between the atmosphere, biosphere, and hydrosphere.

Climate models are used to analyze the present, project changes in the future, and retrace historical events

Climate models are used to analyze the present, project changes in the future, and retrace historical events. For instance, scientists can enter previous greenhouse gas concentrations, which are known thanks to data kept in small bubbles in old ice, and the model will utilize that data to mimic prior temperatures. They are evaluated using information from natural archives and current climatic data.

The strengths of proxy data and climate models vary. Proxies are observable and quantifiable, and they frequently react to temperature in a predictable way. They are not, however, equally dispersed globally or historically. As a result, reconstructing global, continuous temperatures is challenging. In contrast, climate models span both space and time. Although they are frequently quite accurate, they can never fully represent the climate system.

In our most recent review work, we examined global temperature indicators while evaluating climate theory, proxy data, and model simulations. The Earth’s orbit around the Sun, greenhouse gas concentrations, volcanic eruptions, and the intensity of the Sun’s heat radiation were all factors we carefully addressed in our analysis of natural climate-altering activities.

We also looked at significant climate feedbacks that can affect global temperatures, like changes in vegetation and sea ice. Strong evidence, for instance, suggests that there was greater forest cover and less Arctic sea ice around 6,000 years ago than there was in the 19th century. The Earth’s surface would have become darker as a result, allowing it to absorb more heat.

Natural records typically reveal that, compared to the 19th-century median, the average temperature of Earth was around 0.7 C (1.3 F) warmer about 6,000 years ago

Regarding the Earth’s temperature trend over the 6,000 years prior to the onset of present global warming, our two sets of evidence provide contrasting insights. Natural records typically reveal that, compared to the 19th-century median, the average temperature of Earth was around 0.7 C (1.3 F) warmer about 6,000 years ago, before gradually cooling until the Industrial Revolution. The majority of the data we examined supports this conclusion.

In the meantime, most climate models indicate a minor warming trend, which is consistent with a slow rise in carbon dioxide during the millennia following the retreat of the Northern Hemisphere’s ice sheets.

Our analysis identifies a few approaches to enhance climate predictions. For instance, we discovered that models would be more effective if they more accurately captured specific climate feedback. Contrary to most other model simulations, which do not incorporate this enlarged vegetation, one climate model experiment that included increased plant cover in select locations 6,000 years ago was able to mimic the worldwide temperature peak we find in proxy records. As scientists continue to advance our capacity to forecast future changes, it will be crucial to comprehend and more effectively take into account these and other feedback.

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