Over the past century, the Earth's average temperature has swiftly
increased by about 1 degree Celsius (1.8 degrees Fahrenheit). The
evidence is hard 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 by
releasing heat-trapping carbon dioxide from fossil fuels?
Back then, was Earth's temperature warming or cooling?
Even though scientists know more about the most recent 6,000 years
than any other multimillennial interval, studies on this long-term
global temperature trend have come to contrasting conclusions.
To try to resolve the difference, we conducted a comprehensive,
global-scale assessment of the existing evidence, including both natural
archives, like 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 feedback.
Global Warming In Context
Scientists like us, who study past climate, or paleoclimate, look for
temperature data from far back in time, long before thermometers and
satellites.
We have two options: We can find information about past climate
stored in natural archives, or we can simulate the past using climate
models.
There are several natural archives that record changes in the climate
over time. The growth rings that form each year in trees, stalagmites
and corals can be used to reconstruct past temperature. Similar data can
be found in glacier ice and in tiny shells found in the sediment that
builds up over time at the bottom of the ocean or lakes. These serve as
substitutes, or proxies, for thermometer-based measurements.
For example, changes in the width of tree rings can record
temperature fluctuations. If temperature during the growing season is
too cold, the tree ring forming that year is thinner than one from a
year with warmer temperatures.
Another temperature proxy is found in seafloor sediment, in the
remains of tiny ocean-dwelling creatures 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 gets buried by other debris over time, forming layers of sediment
at the ocean floor.
Paleoclimatologists can then extract sediment cores and chemically
analyse the shells in those layers to determine their composition and
age, sometimes going back millennia.
Climate models, our other tool for exploring past environments, are
mathematical representations of the Earth's climate system. They model
relationships among the atmosphere, biosphere and hydrosphere to create
our best replica of reality.
Climate models are used to study current conditions, forecast changes
in the future and reconstruct the past. For example, scientists can
input the 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 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 around the world or through 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 skilful, they will never capture every detail
of the climate system.
A Paleo-Temperature Conundrum
In our new review paper, we assessed climate theory, proxy data and
model simulations, focusing on indicators of global temperature. We
carefully considered naturally occurring processes that affect the
climate, including long-term variations 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 can influence global temperature. For example,
there is strong evidence that less Arctic sea ice and more vegetation
cover existed during a period around 6,000 years ago than in the 19th
century. That would have darkened the Earth's surface, causing it to
absorb more heat.
Our two types of evidence offer different answers regarding the
Earth's temperature trend over the 6,000 years before modern global
warming. Natural archives generally show that Earth's average
temperature roughly 6,000 years ago was warmer by about 0.7 C (1.3 F)
compared with the 19th century median, and then cooled gradually until
the Industrial Revolution. We found that most evidence points to this
result.
Meanwhile, climate models generally show a slight warming trend,
corresponding to a gradual increase in carbon dioxide as
agriculture-based societies developed during the millennia after ice
sheets retreated in the Northern Hemisphere.
How To Improve Climate Forecast
Our assessment highlights some ways to improve climate forecast.
For example, we found that models would be more powerful if they more
fully represented certain climate feedbacks. One climate model
experiment that included increased vegetation cover in some regions
6,000 years ago was able to simulate the global temperature peak we see
in proxy records, unlike most other model simulations, which don't
include this expanded vegetation.
Understanding and better incorporating these and other feedback will
be important as scientists continue to improve our ability to predict
future changes.
(Published under Creative Commons from The Conversation: By Ellie Broadman, University of Arizona, and Darrell Kaufman, Northern Arizona University)
