Earth’s temperature – that is, average surface temperature – is 15°C (59°F). Due to climate change, the Earth has warmed by one whole degree Celsius, since the period 1850-1900 – the pre-industrial baseline level. 1 2 Is this significant, and if so, should we worry? Answer: Yes, on both counts.
Why is a 1 degree rise in temperature significant? A one-degree global change is important because it takes a colossal amount of heat to warm the land, sea, and atmosphere of Earth by that much. For instance, the Intergovernmental Panel on Climate Change (IPCC) forecast that a half-degree (Celsius) increase in temperature by 2100 will lead to a 2 feet 6 inches (76 cm) rise in sea levels, with catastrophic results for tens of millions of coastal inhabitants. It might even prove to be a tipping point for further irreversible climate changes. 3 In other words, a small rise in temperature can have a significant impact on the ecosphere of our planet. For more on this point, see: Why Does A Half-Degree Rise in Temperature Make Such a Difference to the Planet?
- Where Does Earth’s Heat Come From?
- What Determines The Earth’s Temperature?
- What Factors Influence Global Temperature
- What Are The Most Extreme Temperatures On Earth?
- What Is The History Of Temperatures On Earth?
- How Hot Is Earth Compared To Other Planets?
- What Is The Average Temperature Of The Atmosphere?
- How Is Earth’s Temperature Measured?
Where Does Earth’s Heat Come From?
The distance from Earth to the Sun is roughly 93 million miles (150 million km). This puts Earth comfortably within the so-called “Goldilocks Zone”, the habitable zone around the Sun. 4 5 This zone (not too hot, not too cold) lies between 35 million miles and 930 million miles from the Sun. According to climate science, if a planet was any closer, a runaway greenhouse effect (blocking thermal radiation from the planet and so stopping it from cooling down) would vaporize its water reservoir; any further away and no greenhouse effect could prevent its surface from freezing. Although other planets, like Mars, also occupy the habitable zone, the Earth is the only known planet to support life in our solar system.
As the name solar system suggests, the Sun is (literally) the star performer in our corner of the universe. Why is the sun so important? Because without it there would be no energy, no heat and no life. 6
All the Earth’s external energy comes from the Sun in the form of sunlight. This sunlight takes roughly 8 minutes to travel the 93 million miles to Earth. When it does, roughly 30 percent is reflected back into space by clouds and other things. The rest is absorbed by the Earth’s surface and atmosphere and in the process is converted into heat.
In addition to heat from the sun, the Earth also produces its own heat from deep within its core some 4,000 miles beneath the crust. Earth’s core is believed to exceed temperatures higher than the surface of the sun – over 7,000°C (12,600°F). 7 8 This heat comes from two sources: residual energy left over from the formation of the planet nearly four and a half billion years ago; and nuclear energy from natural radioactive decay of uranium and other radioactive elements. A small part of this heat (about 10,000 times smaller than the power Earth receives from the Sun) flows continuously to the surface of the Earth.
What Determines The Earth’s Temperature?
Balance Of Energy
To maintain a stable global temperature, there must be a balance between the heat received from the Sun (together with heat from Earth’s core) and the energy escaping into space. If more energy arrives than leaves, the Earth will heat up. If more energy leaves than arrives, the Earth will cool. To repeat: the energy emitted by the Earth needs to balance both the energy input from the Sun and from Earth’s interior. If this happens, the Earth’s temperature remains constant. 9
Earth’s Energy Budget Prevents Overheating
Earth’s energy budget helps to prevent the planet from overheating due to incoming sunlight. It does so by constantly radiating solar energy back into space. How much energy is radiated? According to the Stefan-Boltzmann Law, the amount of heat a surface (like the Earth) radiates is proportional to its temperature. If, for instance, Earth’s temperature doubles, radiated energy increases 16 times. This heat loss – called radiative cooling – is the primary mechanism that prevents runaway heating on Earth.
Greenhouse Effect Keeps The Planet At A Cosy Temperature Of 15 Degrees Celsius
In fact the Earth emits considerably more energy than it receives from the sun. This is because Earth receives solar energy only during the daylight hours, but emits heat throughout the day and night. Normally, this would lead to a drop in Earth’s temperature except that, fortunately, most of the heat it emits is trapped by greenhouse gases in the atmosphere. This “greenhouse effect” is what maintains Earth’s temperature at a cosy 15°C (59°F). Without greenhouse gases, the average temperature on the surface of the planet would be about minus 18°C (0 °F).
What Factors Influence Global Temperature
Temperatures on the surface of Earth vary enormously because of several factors.
Time Of Day
Earth rotates on its axis once each day (approximately 24 hours). This means one or other side is always in darkness. As a result, temperatures rise in the day and drop in the evening, sometimes substantially.
Time Of Year
Earth has an inclined axis (tilted roughly 23° towards the Sun’s equator), which means that the planet’s Northern and Southern Hemispheres are either tilted towards or away from the Sun during the summer and wintertime, respectively. This means (1) That the hemispheres experience quite different temperatures in (say) July or December. (2) The sun’s rays strike the north and south poles at an angle, whereas the same amount of sunshine hits the equator regions at a higher angle and in a more concentrated manner. This is also why the northern Tropic of Cancer and the southern Tropic of Capricorn are hotspots – the sun is directly overhead during their midsummers. 10
All sorts of climatic conditions can affect temperature. Deep ocean currents propelled by thermohaline circulation, such as the North Atlantic Deep Water (NADW) or the Antarctic Circumpolar Current (ACC), transport heat energy around the globe, influencing local land and sea temperatures. For more, see: How Do Oceans Influence Climate?
Wind currents, like the Jet Stream also have an influence on temperature. Some areas are colder because of denser cloud cover caused by topography, air temperature, and humidity. Clouds themselves are hugely important – see: How Do clouds Affect Climate?
Albedo, the reflective capacity of an object, also has an effect on the amount of heat received by the planet, and thus affects temperature. The more snow and ice, for example, the more sunlight is reflected back into space and the cooler the Earth. This is why the cryosphere is so important to Earth’s climate: if there were no ice sheets or sea ice, the planet would be considerably warmer.
Enhanced Greenhouse Effect
Although the natural greenhouse effect has kept Earth’s temperature at a cosy 15°C (59°F) , or thereabouts, for thousands of years, a major change occurred during the 18th century, ushering in what is now sometimes called the enhanced greenhouse effect.
This began during the period 1750-1840, when humans started burning huge amounts of coal in order to power the new furnaces, machines and engines of the Industrial Revolution. Later, other fossil fuels like petroleum and natural gas were also burned to produce power, for similar purposes. These fossil fuels generated significant amounts of greenhouse gas emissions, which effectively unbalanced Earth’s climate system, leading to the rising temperatures that we now call climate change.
What Are The Most Extreme Temperatures On Earth?
As mentioned, the average surface temperature on Earth is roughly 15°C (59°F), but this bears no resemblance to current extremes.
- The highest ever surface temperature is 70.7°C (159°F). This was recorded by a Landsat satellite in 2004 and 2005 over the Lut Desert in Iran. Part of a global temperature survey conducted by scientists at NASA’s Earth Observatory during the period 2003-2009. The Lut Desert was the hottest spot on Earth from 2004-2007, and also in 2009. The hottest air temperature was recorded at Furnace Creek in Death Valley, California, where it reached 56.7°C (134°F) on July 10, 1913.
- In contrast, according to the World Meteorological Organization (WMO), the coldest temperature ever recorded on Earth was minus 89.2°C (minus 128.6°F) measured at the Soviet Vostok Station on the Antarctic Plateau. The coldest inhabited place on the planet is Oymyakon, in Siberia, where the mercury falls to an average of minus 45°C (minus 49°F) and once hit a low of minus 71°C (minus 96°F). 11
What Is The History Of Temperatures On Earth?
Actual temperature measurements only go back a couple of hundred years. Fortunately, there are other methods we can employ to give us reliable estimates of the Earth’s temperature in the more distant past. These include: ice cores, rock sediment sampling and tree rings.
In The Beginning
The Earth was formed roughly 4.5 billion years ago. Initially it had a surface of molten lava but eventually it cooled sufficiently to form a crust. Land masses appeared and then oceans. About 2.5 billion years ago, oxygen appeared in the atmosphere due to the evolution of cyanobacteria, which produced oxygen as a bi-product of photosynthesis. This new atmosphere made the Earth much colder – average temperatures at the equator are believed to have been on a par with those of today’s Antarctica.
Extinction Of The Dinosaurs (66 Million Years Ago)
About 66 million years a massive comet 6-9 miles wide smashed into Chicxulub on the Yucatan peninsula in Mexico. This resulted in a huge explosion of dust and other aerosols that blotted out the sun for a year, inhibiting photosynthesis and causing Earth’s temperature to plummet. All this led to a global catastrophe during which some three-quarters of the plant and animal species on Earth, including the dinosaurs, were extinguished. In addition to global firestorms, severe long-term geochemical and climatic disruptions devastated the ecology. It took at least ten years for the dust and aerosols to dissipate, while freezing temperatures lasted for approximately three years. This event is a classic example of the cooling effect caused by aerosol deflection of sunlight. 12
Thermal Maximum (55 Million Years Ago)
A major episode of global warming of between 5-8°C occurred about 55 million years ago. The exact cause is still disputed, but most scientists agree that levels of carbon dioxide in the atmosphere rose dramatically – up to 3 times previous levels. This may have been in the form of methane from either the ocean bed or from crystalline water-based solids physically resembling ice, called clathrates. At any rate, scientific data from this period gives us insight into how climate is affected by changing CO2 levels and provides evidence for strong climate sensitivity.
Earth Cools (35 Million Years Ago)
After the thermal maximum, Earth temperatures declined leading to a period of cooling. The first glaciers formed on the tops of mountains in Antarctica. About 14 million years ago, global temperatures dropped by up to 8°C, triggered – it is believed – by the rise of the Himalayas which drew CO2 out of the atmosphere thus reducing the greenhouse effect. This conclusion remains speculative however. 13 The northern hemisphere remained relatively ice-free for more than 10 million years longer.
The Arctic only became significantly glaciated about 3 million years ago, at the beginning of the Quaternary glaciation – an alternating series of glacial and interglacial eras that began about 2.5 million years ago, and is still (technically) ongoing. During this period the ice advanced and retreated to begin with every 41,000 years, then every 100,000 years. Glaciologists think that these varying effects were also influenced by periodic changes in Earth’s orbit that reduce the amount of sunshine reaching the planet. This was then exacerbated by positive climate feedbacks, such as reductions in greenhouse gas levels.
The Ice Age (110,000 BC – 12,000 BC)
Much of our development as anatomically modern humans took place – for reasons yet to understood – during the cooler temperatures of the Quaternary. But Stone Age man survived and then thrived as the thaw led to a dramatic increase in arable land, triggering the advent of modern civilisation.
Broadly speaking, after the end of the last Ice Age came a warm spell, which ended about 3,000 BC. Afterwards, judging by cores drawn from sea beds around the world, global temperatures steadily fell – in total about 0.5°C – until the 20th century. Then they rose abruptly. 14 (See also: When Did Global Warming Start?)
Due to increasing concentrations of CO2 and other greenhouses gases, generated largely as a result of increased industrial pollution and deforestation, average surface temperatures have been rising significantly since 1975. The planet is already 1°C higher than it was around 1900, but according to the Special Report on Global Warming of 1.5°C (SR15) issued by the Intergovernmental Panel on Climate Change (IPCC), by 2100 the average temperature of the planet is likely to be 3°C higher than it was at the beginning of the 20th century.
How Hot Is Earth Compared To Other Planets?
Earth’s temperature is remarkably stable compared to other planets in our Solar System. Temperatures on Mercury, for example, range all the way from molten lava-like hot to freezing cold. This is due to its proximity to the Sun (36 million miles away; 58 million km), slow rotation and lack of an atmosphere to produce a greenhouse effect. On the side facing the sun, temperatures can exceed 450°C on the side, while falling to minus 185°C on the side in shadow.
Venus (68 million miles from the sun; 108 million km), partly due to its dense atmosphere of CO2 and sulphur dioxide, is the hottest planet in our Solar System. Temperatures regularly reach a scorching 460°C or higher.
On Mars (143 million miles from the sun; 228 million km), the average surface temperature is a frosty minus 55°C, varying from a bone-chilling minus 150°C at the poles, to a balmy 20°C at midday on the equator. But mostly, due to its thin atmosphere being unable to trap heat via the greenhouse effect, temperatures are much lower than on Earth.
The average temperature of Jupiter (486 million miles from the sun; 778 million km) is about minus 178°C. Note however that it not made of solid rock but rather clouds of gas. Furthermore, due to Saturn’s tilt in relation to the sun, its northern and southern hemispheres experience seasonal temperatures.
Uranus (1.75 million miles from the sun; 2.8 billion km) is the coldest planet in our Solar System, yielding record lows of minus 224°C. Equally chilly is Neptune’s (3.43 billion miles from the sun) whose upper atmosphere can reach minus 218°C.
Looking at these extremes of temperature, it’s no wonder that we haven’t found life elsewhere in our solar system. 15
What Is The Average Temperature Of The Atmosphere?
Earth’s atmosphere has five distinct layers, all of which enjoy a range of temperatures. The coldest temperatures are found approximately 50 miles (80 km) above the surface. The warmest temperatures are recorded in the thermosphere, due to the strong ionizing radiation it receives at the level of the Van Allen radiation belt, first encountered around 440 miles (640 km) above the earth’s surface. Note: records of atmospheric temperatures date back to the start of the satellite era in 1979.
Rising from the Earth’s surface to a height of 7 miles (12 km) the troposphere has a temperature range of between 15°C (59°F) near the surface, to minus 51°C (minus 60°F) at the top.
Sitting above the troposphere, the stratosphere lies roughly 7 to 31 miles (12 to 50 km) above the Earth’s surface. The stratosphere’s temperature ranges from 51°C (minus 60°F) at its foot, to minus 15°C (5°F) at the top.
On top of the stratosphere, the mesosphere extends roughly 31 to 50 miles (50 to 80 km) above the Earth’s surface. Temperature decreases with height in the mesosphere – from minus 15°C (5°F) at its foot, to minus 120°C (minus 184°F) at its top.
Above the mesosphere, the thermosphere lies roughly 50 to 440 miles (80 to 700 km) above the Earth’s surface. The thermosphere’s temperature increases with height and ranges from minus 120°C (minus 184°F) at its foot, to 2,000°C (3,600°F) at its top.
The exosphere, the outermost layer of the atmosphere, extends as far as 6,200 miles (10,000 km) above the surface of the Earth. The exosphere’s temperature varies enormously from 0 to over 1700°C.
Near Earth Space
Just beyond Earth’s upper atmosphere, there is no atmosphere to reflect the incoming solar radiation. So, on average, near-Earth space measures 120°C (248°F) or higher, while on the shaded side of Earth near-space drops to temperatures of minus 100°C (minus 148°F) or lower.
The average temperature of empty space between the celestial bodies is measured at about 3 kelvins (minus 270.15°C or minus 457°F). Absolute zero, at which all activity stops, is zero kelvins (minus 273.15°C or minus 459.67°F), but in practice the temperature of deep space never falls below than a minimum of just 2.7 Kelvin or -270.45°C (-454.81°F), due to the influence of cosmic microwave background radiation, which permeates the entire Universe.
How Is Earth’s Temperature Measured?
- In order to get a complete picture of Earth’s temperature, scientists collect a comprehensive set of measurements from the air above land and above the ocean surface taken by satellites, as well as weather ships and buoys. 16
- Typically, the measurement at each data point is compared daily with what is ‘normal’ (e.g. 30-year average) for that particular location. The differences, known as “anomalies”, help scientists to form a view of how temperature levels are changing. A negative anomaly indicates the temperature is below average; a positive, that the temperature is above average. Daily anomalies are usually combined to obtain a monthly average which is then used to generate a level of temperature changes from season-to-season and year-to-year.
Today, scientists rely on highly complex analytical software systems for their studies of global temperature. The three most heavily cited combined land temperature and sea-surface temperature (SST) data sets are: (1) the GISTEMP series from the NASA Goddard Institute for Space Sciences (this has the widest coverage, measuring over 99 percent of the globe; (2) the MLOST system produced by the National Oceanic and Atmospheric Administration (NOAA); and (3) HadCRUT4 produced by the UK Met Office Hadley Centre and the University of East Anglia’s Climatic Research Unit.
Once all measurements are registered, a very accurate picture of the global temperature can be obtained by dividing the Earth up into grid-like boxes, calculating the temperature for each box by combining data from all the data points located inside, and then adding up the results. By combining the boxes, researchers work out the temperatures for the northern and southern hemispheres, and then, by taking the average of the two hemispheric values, they can estimate the global average temperature.
At present, all three datasets show a clear trend of global warming although there are year-to-year differences as well. These are caused largely because each system has a slightly differing way of handling certain statistical questions – example, what to do about the lack of data from certain extremely desolate areas?
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