The evidence of climate change

The evidence of climate change

(Image: Cody Whitelaw – Svínafellsjökull, Iceland)

How to start an Ice Age!

Summary

  • In geological terms, we’re currently still in an ice age called the Quaternary Period that began ~2.6 million years ago.

  • We’re also in the Holocene Epoch, an interglacial that began ~11,700 years ago.

  • Confused? Don’t be. Check out this quick primer.

  • The exact timing and relative contributions of the chain of events that led to the current ice age are somewhat uncertain, but scientific evidence shows multiple climate forcings contributed.

  • Once events were set in motion, through feedbacks and tipping points, these forcings compounded one another like a slow-motion toppling of dominoes that began some 100 million years ago (Video 1).

Timeline of events

  • Around 100 million year ago, the Indian tectonic plate left the supercontinent Gondwana. When it collided with the Eurasian plate 35-50 million years later, it pushed up the steep Tibetan Plateau and formed the Himalayan Mountains (Video 1).
  • These new steep mountains were chemically weathered by rain. This is because carbon dioxide (CO₂) in the atmosphere mixes with rainwater (H₂O) to become a weak carbonic acid (H₂CO₃).
  • While the rainwater was only slightly acidic, over millions of years it was enough to gradually erode the mountains. The CO₂ was locked inside river waters as calcium carbonate (CaCO₃) that flowed down into the sea.
  • With increasing amounts of CO₂ being taken out of the atmosphere, more heat from the sun was able to escape back into space because of the greenhouse effect.

Video 1: The movement of the tectonic plates played a crucial role in enabling the current ice age. This short video clip is a time series showing position of continents in the present. It then goes back in time 100 million years before returning to the present.

  • Over millions of years iron and other nutrients were blown and washed down from the landwhich was slowly becoming colder and drierand into the ocean. Here, the iron fertilised tiny free-swimming short-lived microscopic plants called phytoplankton, enabling them to reproduce in extraordinary numbers, taking CO₂ out of the atmosphere (Fig. 1).
  • Phytoplankton are primary producers; everything in the ocean feeds on them either directly or by bigger marine creatures eating the smaller animals that graze on them.
  • When they died, they fell to the deep ocean floor and were eventually buried. This locked the carbon away so it couldn’t enter the atmosphere (see the carbon cycle).
  • Around 200 gigatonnes of CO₂ was eventually removed from the atmosphere over this period, possibly due to this process.
Fig. 1: As iron in the ocean increased (red line), it fertilised the phytoplankton, which withdrew large quantities of CO₂ (blue line) from of the atmosphere.
  • While this was happening, around 34-20 million years ago, both the South American and Australian tectonic plates also separated from Gondwana and headed north (Video 1).
  • This left the Antarctic plate isolated. Ocean currents that once flowed between the tropics and the poles, transferring heat to Antarctica, were partially blocked by a newly forming Antarctic Circumpolar Current (ACC)(Fig. 2).
  • Isolated from the warm waters, Antarctica grew colder and colder. Snow turned into ice that became glaciers which began spreading out over the continent as ice sheets. (Today, the ACC helps to act as a planetary thermostat, keeping Antarctica cool).
  • By 2.6 million years ago, the South American plate had joined the North American plate, separating the Pacific and Atlantic oceans from one another by closing the gap at modern-day Panama, further isolating Antarctica. The glaciers there merged and became an ice cap. The joining of South and North continental plates also changed how the Northern Hemisphere thermohaline current worked, cooling the northern parts of Eurasia and America. By then the Himalayan Mountains were also covered in snow and glaciers. The increased albedo created a feedback effect(2), cooling the area further. Glaciers and ice sheets grew into ice caps and spread out over Eurasia and North America. The Quaternary Ice Age had begun.
  • The timing of the Milankovich cycle amplified the effects of cooling. Some scientists believe it was the primary climate forcing that led to the initial cooling, which was reinforced by subsequent events outlined above.
Fig. 2: A satellite view over Antarctica reveals a frozen continent surrounded by icy waters. The sea ice extent is in light blue. Moving northward, away from Antarctica, the water temperatures rise slowly at first and then rapidly across a sharp gradient. The Antarctic Circumpolar Current (ACC) maintains this boundary. The two black lines indicate the long-term position of the southern and northern front of the ACC (Image credit: The Conversation).

Explainers

Iron fertilisation:

In 1990, oceanographer John Martin hypothesised that iron dust swept from cold, dry landscapes played a crucial role in the ice age by fertilising massive blooms of diatoms and other phytoplankton, using theirs sugar along with copious amounts of carbon dioxide to fuel their growth. Before humans came along, everything that died in the ocean fell to the ocean floor, locking away much of this carbon (see the carbon cycle).

Martin proposed that using iron to trigger diatom blooms might help combat global warming. “Give me half a tanker of iron and I’ll give you an ice age.”

This idea led to several large geoengineering iron fertilization experiments in the ocean. Two of these experiments demonstrated that carbon could be removed to the ocean floor this way. But iron also fertilises highly toxic algae, leading to oceanic dead zones that devastate marine ecosystems, a problem that’s already getting worse because of ocean acidification and increasing ocean temperatures. Moreover the process is costly and far too slow given the rate of climate change. The United Nations Convention on Biological Diversity subsequently placed a moratorium on all large-scale ocean fertilization experiments.

Fig. 3: Large phytoplankton can be seen from space. Photograph off the Canterbury coast January 10, 2008, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

The Albedo Effect:

Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space. The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it much faster than the snow and ice (Fig. 4). As more ice forms, the water is cooler, leading to more ice forming, and so on, in a feedback effect.

Fig. 4 (Photo: NASA/ Nathan Kurtz)