Climate Change Facts and Figures

Climate Change: Facts and Figures

Humans have been disrupting the global climate since the 1800s, through deforestation and burning excessive amounts of coal, oil, and gas. From the 1900s, intensive farming of plants and animals only exacerbated the problem.

What effect has this had on global temperatures and sea levels today? What are the official forecasts for climate change in the 21st century? And how does human activity compare to 500 million years of natural climate change?

Natural Climate Change

Natural climate change usually happens slowly—over tens of thousands of years—to create continuous cycles of cooling and warming. For instance, the Milankovitch Cycles describe changes in the Earth's position relative to the Sun, which cause major climatic shifts between glacial and inter-glacial periods.

Illustration of The Milankovitch Cycles

The Milankovitch Cycles cause continuous cycles of natural climate change. Obliquity (tilt) spans 41,000-year cycles, Eccentricity (orbit) spans 100,000-year cycles, and Precession (wobble) spans 26,000-year cycles.

As a species, we're more or less okay with this because nature gives us a fighting chance to adapt and evolve to the changing climate. Life on Earth has the opportunity to evolve new behaviours and physiology over many generations.

There were four major glaciations during the Pleistocene (aka the Ice Age, ranging from 2.6 million to 12,000 years ago). Each time, global temperatures dropped by a massive 5ºC (9ºF). Many animals, including Homo Sapiens, were forced to undergo huge range shifts to survive.

This had a knock-on effect on our DNA. That's because when a population migrates, there's a heavier expansion load on the gene pool. New environmental challenges—like fewer resources, stiffer competition, and a harsher climate—strike down the weakest individuals first. Only those who can survive in the new environment can pass on their winning genes.

But when climate change happens suddenly, there's no time evolution. And the effects can be catastrophic. Sudden natural climate change is a rare event, occurring just five times in the history of life on Earth. For this, we can blame two wildly destructive natural forces: supervolcanoes and asteroids.


1. Supervolcano Eruptions

Supervolcanoes eject massive amounts of sulphur into the atmosphere very quickly—blocking the Sun, hindering photosynthesis, and yanking hard on the old food chain.

I'm currently sitting about 250km from the Taupo Supervolcano System in New Zealand. It was the most recent supervolcano to blow proper, some 27,000 years ago, with a "smaller" eruption 1,800 years ago.

There are quite a few supervolcanoes on Earth. Yellowstone, for example, last erupted in a big way 630,000 years ago. The Indian Peak-Caliente complex erupted 30 million years ago. Relatively speaking, these super-eruptions are super-rare.

A map of the 14 largest known supervolcano eruptions in the world

A map of the 14 largest known supervolcano eruptions in the world.

2. Asteroid Impacts

The last known impact of an object more than 10km in diameter was 66 million years ago. It hit the Yucatan Peninsula in Mexico, leaving behind the Chicxulub crater which is 150km wide and 20km deep.

The kinetic energy from the impact was converted into explosive energy, producing a blast wave that literally threw soil and rocks into space. The Earth was shrouded in a layer of dust that lasted for months, causing rapid climate change and rendering more than a few dinosaurs extinct.

A map of the 24 largest known asteroid craters in the world

A map of the 24 largest known asteroid craters in the world.

Fortunately, giant asteroid impacts are also rare, such that natural climate change is almost always a slow burn.

Man-Made Climate Change

Now climate change is happening fast, and this time it's all down to us. While politicians and industry fought hard to deny the science of man-made climate change, the science is now undeniable.

"It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred." - IPCC
Illustration of the effects of man-made climate change on Earth

The disastrous effects of rapid man-made climate change.

How does this rapid modern climate change compare to the history of climate change on Earth? Let's look at the past, present, and future trends of two major features: global temperatures and sea levels.

Global Temperature Trends

Since the evolution of complex life on Earth, global surface temperatures have ranged over a massive 10-32°C (50-90°F). This extraordinary data comes from multiple sources, including:

  • Ice cores containing layered bubbles of prehistoric atmosphere
  • Oxygen isotopes stored in ancient marine fossils
  • Fossilised plants whose biodistribution changes with the climate

We now have a record of how global temperatures have shifted over the past 500 million years.

Graph of historic global temperatures on Earth over 500 million years

Historic global temperatures during the last 500 million years. (Data source: Science.org)

At this resolution, global warming doesn't look that bad. But let's zoom in to the Holocene; the 10,000-year period in which human civilisation has thrived.

Graph of historic global temperatures during the Holocene

Historic global temperatures during the Holocene. (Data source: Science.org)

The Holocene was among the most stable periods in Earth's four-billion-year history, with temperatures varying by less than 1°C. Our planet was actually cooling off for the last several thousand years. But the industrial revolution reversed that trend and then some.

Temperatures are now soaring at an unprecedented rate. The average global temperature is now 0.8°C up on the 20th century average.

And it's not stopping. The IPCC predicts that current pledges to slow climate change will still allow temperatures to rise a further 2°C in the 21st century.

That's the best case scenario. The worst case, where our emissions remain constant or even grow, will push global temperatures to 4°C higher than pre-industrial levels.

Graph of global temperature forecasts to 2100

Forecast global temperatures to 2100. (Data source: Carbon Brief)

But there's another variable we need to take into account. And that's the largely unpredictable degree of melting permafrost that could unlock vast deposits of carbon dioxide and methane trapped underground.

These gases come from ancient plant matter that froze before being fully decomposed by bacteria. They've been trapped in land-based permafrost and under the sea bed for thousands of years.

Warmer temperatures will destabilise these gases and release them into the atmosphere, pushing global temperatures even higher in a catastrophic feedback loop.

The IPCC's best case scenario will see 85% of permafrost gases released between now and 2100. Methane has already started leaking from the sea floor in Antarctica.

  • The Good News. Some evidence suggests the liberated methane doesn't make it into the atmosphere. Most of the methane is under the sea bed, so should dissolve into the ocean.
  • The Bad News. The carbon dioxide in the land-based permafrost will reach the atmosphere. Some 1,000-5,000 gigatons of ancient carbon will flood the air this century, releasing 100-500 year's worth of carbon at humanity's already catastrophic rate of emissions.

Once this natural cycle passes a threshold rate, there may be nothing to can do to avoid a complete climate catastrophe.

Sea Level Trends

Global temperature is only one aspect of climate change. We also need to think about sea levels, which have two major influencers:

  • Melting Ice. As the planet warms, glaciers and ice sheets melt into the ocean, raising sea levels. This happens naturally at the end of every glacial period, but right now we're in the middle of an inter-glacial. Man-made climate change has caused the Arctic to lose 40% of its ice in the last 40 years.
  • Ocean Temperatures. Thermal expansion means that warm water takes up significantly more space than cold water, increasing the overall volume of the ocean. Once again this is driven by rising temperatures caused by human activity.

In the past, sea levels were naturally much lower than they are today. Around 10,000 years ago when the last glacial period ended, sea levels gradually rose by 120 metres (400 feet) to shape the coastlines we see today. An equilibrium was reached about 3,000 years ago. Cue most of civilised human history.

But now sea levels are rising again, and it's not part of a natural cycle. Since real-time records began in the late 1800s, we've seen a rise of around 20cm (8 inches).

Graph of historic sea level rise 1880 to 2020

Historic sea level rise from 1880 to 2020. (Data source: US Global Change Research Program)

That may not sound like much, but it's enough to create an existential threat to people living on islands in the Pacific. Eight such islands have already been swallowed whole. In the US, more than 90 coastal cities are already experiencing chronic flooding.

Continual sea level rise will be a steady onslaught for the 250 million people living in coastal regions less than 5m (16 feet) above sea level. In the coming years, storm surges, high tides, and rising oceans will create frequent and serious flooding.

How much will sea levels rise? The major ice sheets of Greenland and Antarctica contain enough ice to raise sea levels by 65m (213 feet). Estimates vary as to exactly how much to expect this century. NASA predicts a rise of 30-240cm (1-8 feet) by 2100, while the IPCC forecasts 80-100cm (2.6-3.3 feet).

Graph of forecast sea level rise to 2100

Forecast sea level rise to 2100 (Rahmstorf et al., 2011)

The problem for your great-grandchildren is that sea levels will continue to rise for many centuries thereafter, even if we completely eliminate our carbon emissions this century.

Becky Casale Author Bio

Becky Casale is a science blogger based in Auckland. If you like her content, please share it with your friends. If you don't like it, why not punish your enemies by sharing it with them?



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