This is a long but thorough reply. I've taken this as an opportunity to lay down a complete explanation of the phenomenon of "Global Warming". I may go back and edit this to add/modify more points later. I'm also planning an answer to What should we do about global warming? but I may take a break for a while.
Sources are at the very end.
The short, and abbreviated answer is: imbalanced heat-budgets. The longer answer is a bit more involved, but it boils down to three core concepts:
- In order to understand global warming you have to understand why we have the stable climate that we have.
- In order to understand our climate stability you have to understand how the water and atmosphere affects the surface temperature of the Earth.
- The easiest way to understand how the water and atmosphere influences the surface temperature is to compare our climate to the moon's.
The Moon's Temperature
The moon is, without meaningful difference, the same distance from the sun as the Earth, and yet:
- The "light side", the side facing the sun, has an average temperature of around 250 Degrees Fahrenheit (F) (121 Degrees Celsius (C)).
- The "dark side" of the moon has an average temperature of -250 F (-157 C).
- This means the difference between day/night temperatures on the moon is 500 F (278 C).
- The poles have been measured at an impressive -413 F (-243 C). That's only 43 F (30 C) above absolute zero and comparable to Pluto's surface!
The Earth's Temperature
Meanwhile, on Earth:
- The hottest recorded place (Death Valley, CA) was 134 F (56.7 C).
- The coldest recorded place (Vostok Station, Antarctica) was -128 F (89 C).
- The average Earth temperature is 61 F (16 C).
- The largest recorded difference between day/night temperature in one location (Loma, MA) is 49 F (9 C) versus -54 F (-48 C) which makes a difference of 103 F (57 C), in the desert.
- Some places can have temperature differences between day/night as small as 7 F (4 C), in humid Hong Kong.
- The Moon's maximum temperature is 116 F (64.3 C) hotter than Earth's.
- The Moon's minimum temperature is 285 F (154 C) colder than Earth's.
- The difference between night/day temperatures on the Moon is 397 F (221 C) greater than even Earth's most extreme case!
All of this while the Moon and Earth sit approximately the same distance from the sun!
Why the difference?
So why are there such huge differences between the Moon/Earth? Why is the Moon's climate so much more extreme than Earth's, with higher highs, and lower lows?
It all boils down to the fact that the Earth is mostly covered by water and has a nice, thick atmosphere that's full of water vapor and other gasses, while the moon is bare rock.
Why is water so important?
Water is a fascinating molecule with some interesting properties, the most relevant here being its amazing capacity for absorbing heat. In fact it has the second highest "heat capacity" of any non-carbon molecule, only behind Ammonia.
What this means is that it takes a tremendous amount of energy to raise the temperature of water compared to other materials, and it takes a long time for that heat to escape and for the water to cool.
Conversely, rock and metal have a very low heat capacity. They heat up quickly, and also cool down quickly. It doesn't take much to get their temperature up, but they also lose that heat easily.
Think about how long it takes to boil water on the stove compared to just heating an empty metal pan. Then think about how long it takes for boiling water to cool to room temperature, while an empty pan off the stove will be cool to the touch in a matter of minutes. The point is, water resists change and retains heat for a very long time. (This is also why water is a highly efficient coolant)
How water's properties affect the climate
Relating this back to the question of the Earth's climate, the water on the surface of the planet serves as an efficient means to resist rapidly increasing/decreasing temperatures. The water also serves as an efficient means of (relatively) long-term heat storage.
Regions of earth that are surrounded by water and have a humid climate tend to have very stable temperatures and rarely experience extreme temperature variance in short periods of time. Regions of earth that are far from water, are very dry, or only have a very small surface area covered by water compared with exposed rock, metal, and sand will tend to have rapid and extreme temperature variations between seasons and between day and night. This is why coastal towns experience a mild climate while inland towns, or especially towns in deserts, tend to experience large variances in temperature and weather.
The simplest evidence of this difference in heat capacity is when you go to the beach and notice how windy it is. The wind is a result of the land and the water changing temperatures at different rates and influencing the temperature of the surrounding air to create a pressure gradient that drives continuous winds from cooler areas to hotter areas. In the daytime the land heats quicker than the water, and the wind blows inland. In the nighttime the land cools quicker than the water, and the wind blows out to sea.
In short, the surface water serves as a local temperature stabilizing force that resists large differences in temperature due to day/night/seasonal differences. It keeps the Earth cool during the day, warm during the night, and locks away the sun's heat so that it's slowly released over time, even if the seasons get colder. This is also why you might get your first snow before the lake freezes over, because the lake has a lot of heat stored away that has to be lost while the water droplets in the atmosphere are small enough to cool quickly.
But what about larger-scale areas beyond the local environment?
In addition to simply serving as a local temperature buffer, water also distributes heat across the surface of the planet. When a fluid, such as water, experiences differences in temperature across its surface it also experiences differences in density in these regions which then result in a net movement of the fluid from denser regions to less-dense regions, and the movement of the denser regions away pulls in water to fill the vacated space. The net result is that currents form and these result in the transport of high temperature water into colder areas, and low temperature water into warmer areas, creating a further mechanism for equilibrating temperature over large scales.
But what about regions that are far away from water?
Why is the atmosphere important?
One unfortunate quality of water (from a global temperature regulation standpoint) is that, at our planet's temperature, it is a liquid. A liquid can only occupy a fixed volume of space, and is unable to extend beyond that volume. This is only unfortunate in the sense that it means that only regions that are close to a body of water can reap the temperature regulating benefits provided by said water body. Regions that are further away are less able to benefit from the water body's temperature-regulating effects.
On the other hand, gasses expand to fill the available volume of space. Therefore, even in places on earth that don't have directly adjacent bodies of water, there is still an overlying atmosphere composed of gasses. In other words, the atmosphere facilitates temperature regulation in regions that are far from water.
Gasses allow for some temperature regulating benefits at the surface, but not nearly to the extent of water. Most gasses have a far lower heat capacity than water, so they're not as efficient at directly absorbing heat as water, but it's still better than nothing. The gasses of the atmosphere provide a less efficient direct temperature stabilizing force than water bodies, but this isn't their primary means of regulating temperature.
The atmosphere extends water's influence
In addition to the gas molecules themselves, the atmosphere provides a medium in which water can vaporize and distribute across the entire surface of the planet. Water vapor usually is found near bodies of water from which it evaporated, but it can be transported even into regions far flung from water, given the right conditions. It doesn't rain much in the desert, but it does rain sometimes.
Water that's vaporized exhibits many of the qualities of liquid water, except that its ability to resist temperature changes and store heat are reduced compared to bodies of liquid water. Therefore, vaporized water can transport heat like the seas, but it's less efficient. Also, when atmospheric water collects and cools it precipitates, adding water to places that may be far from the coasts, and providing water's temperature-regulating benefits inland while transporting the absorbed heat back into the atmosphere or, eventually, into the sea.
This whole process is called the Water Cycle, and in addition to explaining the means by which water moves through a system, it also explains how water can have influence over the climate of inland locations far from coastal regions. Places with low water content and little water inflow are deserts and exhibit desert-like properties including extreme temperature variations.
However, the primary reason why we're concerned with the atmosphere isn't it's ability to transport heat directly. So what is the primary effect that the atmosphere plays regulating the Earth's temperature? To understand that we need to understand how celestial bodies lose heat.
How heat is lost by celestial bodies
Bodies in a vacuum (such as the Earth, or the Moon) have only one means to gain or lose heat: thermal radiation. All other means of heat transfer require direct contact between bodies.
Thermal radiation is a result of a material with high temperature emitting black body radiation (radiated heat) where the wavelength and intensity of the radiation depend on the temperature of the object. For example, a hot iron or electric stove will turn red as it gets hot, and the coils on a toaster can toast bread without directly touching it.
This is a result of thermal radiation leaving a heat source (the toaster coils, or the sun) and being absorbed by a surface (the toast, or the Earth). The end result is a rise in temperature by the absorbing surface that eventually drops as the surface emits its own thermal radiation and loses heat.
How the Atmosphere Interferes with Heat Loss
The primary means by which the atmosphere warms the planet is by slowing the loss of heat. When thermal radiation strikes the earth it could be absorbed by gasses/vapor in the atmosphere, absorbed by a body of water, or absorbed by a solid surface. This raises the temperature of the surface, which then emits its own thermal radiation as it loses heat.
When thermal radiation is emitted by a surface within the atmosphere, the radiation either leaves the planet and travels into space, which cools the planet, or it is reabsorbed into the atmosphere or water vapor.
The thicker the atmosphere, the more water vapor present, and the more heat-absorbing molecules (such as CO2 and CH4) that are present in the atmosphere, the harder it is for heat to escape, and the more heat that is retained.
This is called the "Greenhouse Effect" because it's the same principle that keeps a greenhouse hotter than the open surroundings, and is the primary reason why the Earth is warmer than the moon.
To lay out the situation let's look at the simple system of the moon as an example for comparison.
Heating and Cooling the Moon
When thermal radiation from the sun hits the moon, it is absorbed as heat or reflected as light (why the moon shines). The moon loses heat by emitting thermal radiation in the form of infrared light which is emitted into space as the surface of the moon cools. This process is rapid and there are no complicating factors.
Radiation is absorbed, temperatures rise. Radiation is emitted, temperatures fall. Temperatures are extremely localized and there is no means of distributing them. Heat is not quickly distributed from one location to another, each region's temperature is independent of all other regions unless they're directly touching.
Heating and Cooling the Earth
When thermal radiation from the sun hits the Earth, it's more complicated.
Infrared radiation is absorbed or reflected by the atmosphere, water vapor, surface water, or land and temperatures rise. Some radiation is emitted back into space and the temperature falls. Some heated particles join air/water currents that disperse the heat over a large area, which spreads heat absorbed in one place to distant regions that weren't directly affected, and slows the rate at which thermal radiation is emitted by slowing the rise in temperature of any small area. This allows more heat to be absorbed than emitted while the temperature is low.
Some heated particles emit infrared radiation only to have it reabsorbed elsewhere before it can escape back into space. Sometimes radiation makes many "jumps" before it finally escapes, and these "jumps" allow the heat to be retained for a very long time after their initial arrival and causes the atmosphere to warm which in turn can warm the land and sea.
The atmosphere makes it harder for heat to escape once it's entered, and water stores a lot of heat before its temperature starts to rise, so a lot of heat gets captured and retained before it starts to get lost to space.
Also, since the water and atmosphere are so good at distributing the heat at short and long distances it means the heat is spread out over a much wider area than it first arrived and is only slowly given up by the water in the system. This means that heat is retained over days/nights/seasons, and is spread by currents to even out the differences in heat input in different regions of the Earth. This setup doesn't produce a homogeneous planet where everything's the same temperature, but it does provide a much more even distribution of heat than the moon which lacks these systems.
There's just one last point left to understand Global Warming, and it's a culmination of applying all of the above concepts to a time scale where we measure their effects. This one last concept is called heat budgets.
A heat budget is much like a regular budget, except instead of money you're looking at heat exchanges.
In a regular budget you lay out all the ways that money enters a system and the amounts that enter from each means (income). You also lay out all the ways that money exits a system and the amounts that exit through each means (expenses). You sum up the total income (gross income) and you sum up the total expenses (gross expenses) then you subtract the gross expenses from the gross income to find out whether you've gained money (net profit) or lost it (net loss). At the end of the day you apply your net profit/loss to how much money you started with (your bank balance) to find out how much money you've now got in total.
A heat budget is exactly the same, except:
- money = heat
- income = incoming heat = from the sun
- expense = outgoing heat = thermal radiation that escapes the Earth
- gross income = total incoming heat
- gross expenses = total outgoing heat
- net profit/loss = net heat increase/decrease
- bank account = global temperature
Here's a good diagram of what the Earth's heat budget looks like at present (by NASA):
Conclusion: How it all fits together
The issue of Global warming is that prior to the industrial revolution the Earth's heat budget was in a reasonable state of equilibrium (natural slow million-year scale shifts aside) where the amount of heat coming in to the Earth's atmosphere was equal to the amount of heat leaving the atmosphere, so the global temperature wasn't really changing much.
The problem that arose, starting with the industrial revolution and continuing to this day, is the fact that in order to facilitate the massive-scale productivity that allowed us to produce large amounts of goods for sale to hugely increase the global economy, we have to burn a large quantity of oil and coal which put a lot of extra Carbon Dioxide (CO2) into the atmosphere. We also release natural gas (CH4) into the atmosphere by accident or as a result of livestock production or other mechanisms. We also remove a lot of natural systems that remove CO2 from the atmosphere, such as forests, jungles, swamps, marshes etc. due to expansion of living areas, conversion of land use, harvesting resources, or simply due to carelessness.
The problem, then, is that through the course of our actions, and over the long term, we are fundamentally altering the composition of our atmosphere in such a way that the rate at which CO2 and CH4 is entering the atmosphere is increasing, while the rate at which it's removed is decreasing. This increase in concentration makes our atmosphere more efficient at retaining heat and reduces the amount of heat that can be lost to space. This means the temperature of the earth will rise as a result of being unable to efficiently cool, and it means that the environment in which we live will change. The manner in which specific weather sytems, such as your local area, will change is unpredictable as the Earth is efficient at distributing heat (as noted above). All that can be accurately said is that the average temperature of the Earth will continue to go up until it is high enough that the amount of heat lost is equated with the amount of heat being continuously gained.
It's important to note that there's a lag time between an increase in CO2 and CH4 in the atmosphere and an increase in temperature. The rate at which heat is entering the Earth's biosphere isn't changing (the sun isn't getting any hotter), all that's changing is that we're not cooling off as efficiently as we used to. A good analogy is that it's like throwing on a blanket: it doesn't heat you up immediately, but your maximum temperature goes up once you warm up to it. What we're effectively doing right now is piling on more blankets every year, but it will be a while before we see the temperature catch up to the maximum threshold we've set up for ourselves.
The terrifying danger is that by the time we reach a temperature that we realize we need to fix the situation, we will be too deep to get out before we're cooked.
Worst Case Scenario:
The temperature of the planet reaches a "tipping point" where the increase in temperature suddenly flies out of control and the planet suddenly gets very hot very fast. This has happened before, and one of the single biggest dangers is that if the ice caps melt completely it may trigger it happening again a rapid rise in temperature would result from a reduced reflectivity of the earth and resulting in more sunlight getting absorbed than previously, producing a positive-feedback loop where the Earth rapidly "cooks" itself.
This would almost certainly result in a mass extinction of all species that are adapted to their current environments as it would alter every environment on earth short of the deep sea, but even then the acidification of the oceans (more CO2 in the atmosphere causes it to dissolve into the oceans raising the acidity of the oceans and killing off much of the life there) may kill off most ocean life as well. That would result in a mass extinction where humans wouldn't stand a chance at survival.
At that point intelligent life on earth would have to evolve again, but then what would be the point? Intelligent life will have already proved to be an unsustainable outcome.
Well, I think we had a pretty good run, at least. Maybe the next intelligent race to inhabit the Earth will think kindly of us? Though I'm not sure how much consolation that will be to most people.
What Are the Causes of the Extreme Temperature Differences on the Moon? | The Classroom | Synonym
What is the Temperature on Earth?
Diurnal temperature variation
Properties of water
Earth's energy budget
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