12.3: Past Climate Changes

The Last Two Millennia

Using proxies and historical accounts, such as pictures of the frozen Thames, as illustrated in Fig. \(\PageIndex{1}\), scientists have obtained a historical record of Earth's climate. Over the last two millennia, proxy temperature and carbon dioxide measurements can be correlated with records of human art and architecture. The PAGES 2k initiative was a global project to reconstruct temperature and hydroclimate records for the last 2,000 years, using data from a wide range of sources like tree rings, ice cores, and corals.

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Figure \(\PageIndex{1}\): A: A famous painting of a Frost Fair on the frozen River Thames around 1683-84 AD. (Public Domain; Thomas Wyke via Wikimedia Commons). B: Ruins of Hvalsey Church from the Greenland settlements of the Norse. (Public Domain; Number 57 via Wikimedia Commons)

The reconstruction suggested that a short period of relatively cold conditions occurred in Europe from the 16th to the 19th centuries, known as the Little Ice Age. Conversely, relatively warm conditions during the 9^th to 13^th centuries, known as the Medieval Warm Period, may have enabled Vikings to colonize Greenland and reach North America. Some reconstructions of global temperatures, however, indicate that the Medieval Warm Period was not a global phenomenon, as shown in Fig. \(\PageIndex{2}\). These reconstructions also suggest a long-term cooling trend over the past 2,000 years that culminated in the Little Ice Age, followed by relatively rapid warming in the 20^th century. According to the PAGES 2k reconstruction, the global average temperature during the 30 years from 1971 to 2000 was warmer than during any other 30-year period over the last 1,400 years. This suggests that the recent warming is unusual. The rate of change over the last ~100 years also seems unusually fast compared with the previous 2,000 years. The two independent reconstructions agree well in the cooling trend over the past 1,000 years, but the PAGES 2k reconstruction suggests slightly warmer conditions during the first millennium CE (Common Era). Another recent attempt to reconstruct past temperatures is the Marcott et al. (2013) dataset, which is based primarily on lower-resolution ocean sediment cores. It is therefore smoother than the higher-resolution PAGES 2k dataset, which includes land-based data such as pollen and tree rings.

Figure \(\PageIndex{2}\): Global multi-proxy temperature reconstructions from the PAGES 2k project based on seven continental-scale regional reconstructions (green) compared with an independent reconstruction that includes the whole Holocene and the instrumental record (red). The PAGES 2k reconstruction represents 30-year averages. It contains many more data points than the lower-resolution Marcott et al. (2013) reconstruction, particularly for the last 1,000 years. Thus, the error range of the PAGES 2k reconstruction is presumably much smaller than that indicated by the blue shading. (CC-BY-4.0; Klaus Bittermann via RealClimate). Alternative Text Description.

The Holocene

Fig. \(\PageIndex{3}\) shows the whole Holocene (the last 10,000 years) reconstruction of global average temperatures from the Marcott dataset. It suggests that the long-term cooling trend over the last 2,000 years is part of a longer trend that extends back to the middle Holocene around 4,000 BC. The early Holocene, from around 8,000 BC to 4,000 BC, was relatively warm, similar to recent decades. This remains debated in the scientific community, with recent research suggesting that it wasn’t warmer during the early Holocene and that biases in seasonally sensitive proxies are to blame. If this is true, the current warming will be unprecedented for more than 10,000 years, perhaps more than 100,000 years or longer. The rate of temperature change appears to be much slower than it has been over the last 100 years. Still, the reconstruction's relatively low resolution smoothens it and prevents a fair comparison with the instrumental record on 100-year timescales.

Figure \(\PageIndex{3}\): Holocene global surface temperature reconstruction from Marcott et al. (2013, blue) with shaded error ranges together with the instrumental record (red) as a function of time in years CE. (CC-BY-4.0; Klaus Bittermann via RealClimate). Alternative Text Description.

Now let’s examine CO₂. Is the observed increase in atmospheric CO₂ during the last 60 years unusual compared to the pre-industrial Holocene? Ice cores can be used to answer this question. When snow accumulates on an ice sheet, it is compressed into ice by the overlying snow. During this compaction process, small air bubbles are trapped within the ice, as depicted in Fig. \(\PageIndex{4}\). In the lab, this ancient air can be extracted from the ice by mechanical crushing, and its CO₂ concentration, as well as those of other greenhouse gases, can be measured.

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Figure \(\PageIndex{4}\): A: Ice cores have been drilled in different locations in Antarctica by research scientists (Public Domain; NASA via Wikimedia Commons). B: These ice cores are preserved and analyzed to obtain past climate data (Public Domain; Heidi Roop via National Science Foundation). C: When snow falls, tiny bubbles of air get trapped in the ice through compaction of snow. These air bubbles in the ice are visible to the naked eye (CC BY-NC 4.0; R Mulvaney via British Antarctic Survey). E: The air bubbles under a microscope (CC BY-NC 4.0; W Berner via Nature/University of Bern)

Ice cores from Greenland are less suitable for CO2 reconstructions because they are often contaminated with impurities (e.g., dust), which can lead to CO2 production within the ice. However, Antarctic ice is so pure that it provides excellent records of past atmospheric CO₂ concentrations. Multiple ice cores have been drilled across Antarctica to study past climate. The measurements from the youngest ice match up very well with the direct measurements of modern air from Mauna Loa, as illustrated in Fig. \(\PageIndex{5}\). Also, the measurements from different ice cores agree with each other (different colored symbols in Fig. \(\PageIndex{5}\)). This indicates that Antarctic ice cores faithfully record past atmospheric CO₂ concentrations. The results show that atmospheric CO₂ concentrations have been relatively constant at about 260-280 ppm during the Holocene. It was only during the last 200 years that CO₂ concentrations started to increase. Thus, we conclude that the increase in CO2 over the last 200 years is unprecedented and has not occurred during the preceding 10,000 years. We also know that burning fossil fuels increased dramatically after the Industrial Revolution (1760-1840).

Figure \(\PageIndex{5}\): Ice core CO₂ measurements of ancient air. The different colors indicate different ice cores. The inset zooms in on the last 200 years and includes, in red, the modern air measurements from Mauna Loa. Note that the ice core measurements agree well with the modern data where they overlap. (Public Domain; Summary for Policymakers via IPCC 2007). Alternative Text Description.

Other greenhouse gases have also been measured in air extracted from ice cores. Methane (CH₄) and nitrous oxide (N₂O) show a very similar behavior to CO₂, such that their concentrations were relatively constant throughout the Holocene, around 700 ppb and 260 ppb, respectively, and increased dramatically during the last 200 years to values around 1,700 and 310 ppb, respectively (IPCC, 2007).

The Ice Ages

Fig. \(\PageIndex{3}\) already hints at a cold period before the Holocene. Indeed, we now know that Earth had been in an ice age, or glacial state, for a long time before the current Holocene warm period began. But it was only in the 19^th century that scientists realized that Earth had experienced global ice ages. This discovery was made by Louis Agassiz, a Swiss geologist, who hypothesized that not only Alpine glaciers were advanced but also large ice sheets moved south from northern Europe and America, leaving glacial landforms behind, as shown in Fig. \(\PageIndex{6}\).

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Figure \(\PageIndex{6}\): Glacial landforms across the world provide evidence of past glacial advances. A: Polished bedrock with striations indicates a glacier was moving over it. The glacier incorporates rocks into its base and, by pushing them over the underlying bedrock, creates grooves. This example is from Mount Rainier National Park (CC BY 2.5; Walter Siegmund via Wikimedia Commons). C: Moraines are glacial deposits formed at the side (lateral moraines) or end (terminal moraines) of a moving glacier. This example from Svalbard shows rocks and debris left behind by a glacier (CC BY-NC 4.0; Dr. Karen Kleinspehn via MTU). B: Erratic boulders like this one from Scotland, many miles from a possible bedrock source, have been attributed by Louis Agassiz to the action of ice-age glaciers. (CC BY-SA 2.0; Patrick Mackie via Wikimedia Commons)

During the height of the last ice age, the Last Glacial Maximum (LGM) roughly 20,000 years ago, large additional ice sheets covered parts of North America and northern Europe, as illustrated in Fig. \(\PageIndex{7}\). The Laurentide Ice Sheet was more than 3 km thick and covered all of what is now Canada and part of the northern United States, reaching as far south as New York City, Chicago, and Seattle. The Eurasian (or Fennoscandian) Ice Sheet covered all of Scandinavia, much of the British Isles, the Baltic Sea, and surrounding land areas from northeastern Germany to northwestern Russia. Mountain glaciers also descended farther into valleys and often into lowlands. Because so much more water was locked up as ice on land, the sea level was 120 m lower during the LGM than it is today.

Figure \(\PageIndex{7}\): Reconstruction of ice sheet extent near the peak of the Last Glacial Maximum. (Public Domain; Ice sheet extent via Climate.gov)

The LGM is a well-studied period in paleoclimate research, and a wealth of data is available. Ice cores show lower atmospheric greenhouse gas concentrations, such as near 180 ppm for CO2, compared with 280 ppm during the late pre-anthropogenic Holocene. Vegetation reconstructions indicate that forests were replaced by tundra and grasslands across large areas of the mid- to high latitudes. We also know from ice and ocean sediment cores that the air was dustier. Temperature proxies show colder temperatures almost everywhere. However, temperature changes were not uniform across the region. Temperature changes over large parts of the tropical and subtropical oceans were relatively small. Globally averaged sea surface temperatures have been estimated to be only 2°C cooler than present. Land areas in the tropics experienced moderate cooling of about 3°C. The most significant cooling, exceeding 8°C, occurred over land at mid- to high latitudes and over Antarctica. The globally averaged surface air temperature is estimated to have been 4°C lower during the LGM. Studies also suggest that, on average, land cooling was 3 times greater than ocean cooling. The land-sea contrast and polar amplification are similar to those observed during the past century of warming. This suggests that those are robust properties of the climate system. We conclude that paleoclimate data from the LGM indicate that Earth was dramatically different from today, with large ice sheets, low sea levels, and other vegetation. These changes occurred even though global average temperatures changed by only 4-5°C. This is comparable to changes projected for some future scenarios.

Fig. \(\PageIndex{8}\) shows that there were about nine glacial-interglacial cycles during the past 800,000 years. Most of the time, the sea level was lower than today, reaching more than 120 m below sea level during some glacial maxima. Oxygen isotope (δ¹⁸O) measurements in benthic foraminifera from deep-sea sediments indicate the extent to which water was locked up as ice. Higher ice volume will result in lower global sea levels. Ice core records also show that glacial periods were consistently associated with low atmospheric CO₂ concentrations and low temperatures in Antarctica. CO₂ concentrations varied between about 180 ppm during glacial maxima and 280 ppm during interglacials. The correlation between these completely independent datasets, one from Antarctic ice cores, the other from deep-sea or benthic foraminifera, is astounding. It demonstrates that climate and the carbon cycle are tightly interlinked. High CO₂ concentrations are always associated with warm temperatures, high sea levels, and low ice volume. This indicates the importance of atmospheric CO₂ concentrations for climate, but it also suggests that climate influences the carbon cycle and, in turn, alters CO₂ levels.

Figure \(\PageIndex{8}\): This graph illustrates the variations in sea levels (blue), atmospheric CO₂ concentrations from ice cores (black), Antarctic temperature anomalies measured using deuterium ratios measured in the ice (red), and Antarctic productivity (green) as a function of time. (CC BY-NC 4.0; Daniel M. Sigman et al., 2010 via Nature). Alternative Text Description.

A closer inspection of leads and lags shows that during the last deglaciation, CO₂ and Antarctic temperatures lead global temperature and sea levels, suggesting that CO₂ is an important forcing mechanism for the warming. But we also know that climate affects the carbon cycle. For example, CO₂ is more soluble in colder water, which means the ice-age ocean can take up additional carbon from the atmosphere. Another likely reason for the lower atmospheric glacial CO₂ concentrations is iron fertilization. The colder glacial atmosphere was also dustier. Dust contains iron, and thus, iron deposition in iron-depleted regions, such as the Southern Ocean, increased during the ice age. This intensified phytoplankton growth and the biological pump, which are together called Productivity (Figure \(\PageIndex{8}\)). This increased phytoplankton activity in the Southern Ocean intensifies sinking and carbon sequestration in the deep ocean. Recent research indicates that about half (~45 ppm) of the glacial-interglacial CO₂ variations can be explained by temperature, and another 25-35 ppm by ocean iron fertilization. Thus, we can conclude that climate and the carbon cycle are tightly linked.