Article curated by Ginny Smith
Scientists agree that human activities are causing climate change, but there are many unanswered questions about it, from how we measure the past to how we measure the future, to the changes we’ll see in the weather and the impacts we’ll see on plants and animals. The Earth is a complex ecosystem, held in a fine balance that allows life to thrive, and offsetting this balance can have massive repercussions... A relatively small change in temperature can have big knock-on effects, and a large change could result in an uninhabitable Earth.
The Earth is protected from the sun by distance, by the atmosphere, and by clouds and ice which reflect back radiation. If more energy is absorbed than emitted, the Earth warms, and if less energy is absorbed than emitted, the Earth cools. Currently, the absorbed solar energy exceeds the emitted thermal energy by just 0.4% - and this gives us global warming.
Greenhouse gases (mostly carbon dioxide, but not only) absorb infrared radiation and re-emit it in all directions. This causes more heat to be trapped within the atmosphere, and cause the warming known as the ‘greenhouse effect’.
Humans are contributing to global warming and climate change by emitting more carbon dioxide and other greenhouse gases; however, it is difficult to predict the extent of warming and its consequences, and this is currently something we can only make guesses about.
One thing that makes it hard to guess what will happen to the climate is clouds. You only need to feel a cloud to pass across the sun on a hot summer's day to realise hpow big an effect clouds have on the temperature of the Earth. Clouds affect albedo (the proportion of light which gets reflected back into space), but are also made of water, which acts as a greenhouse gas, keeping the heat of the earth trapped in the lower atmosphere. These two opposite effects make the cloud contribution to global warming complicated and hard to measure or predict! Because of this, we do not yet know the net contribution to global climate from clouds.
Even when we have data, guessing the future is hard. This is because we need to extrapolate: and extrapolation is tricky. It involves taking known, reliable data, and guessing at how it behaves outside the measured spectrum – and that is something we just don't know. To extrapolate data, scientists make assumptions that things will continue to behave as they did before (and they might not). To estimate global warming, scientists have to make all kinds of assumptions, from population sizes to dietary patterns round the world to what we’re using to generate energy; there is so much uncertainty in these things that it makes their predictions unreliable. This is why most climate models use three models: “most likely”, “worst case scenario” and “best case scenario”, and compare their results.
One way scientists have tried to measure past temperatures is using fays stored in the pores of ice cores. Algate which live there secrete fats that can be profiled using gas chromatography to work out their structures – and these turn out to be very interesting: the fats have much the same framework structure, but differ in their degree of unsaturation. Just as more saturated fats like butter are harder than margarine, the level of unsaturation changes how runny the algae fats are. In fact, algae always want fats of the same runniness... but when it’s colder the fats set more easily, so they make runnier ones, and when it’s warmer, they make less runny ones. This means scientists can use the leftover algae fats in ice cores laid down during different time periods (and can be time stamped by carbon dating other biological material), to record past temperatures.
Another way to learn about past climates is to study the behaviours and migrations of indicator species such as bats. Bats have a profound effect on biodiversity and are found all round the world, providing a unique opportunity to examine planetary symbiosis. Bats are seed dispersers, material and nutrient distributors (encouraging habitat development and reforestation), pollinators of tropical plants not pollinated by bees, and pest controllers, eating a quarter of their bodyweight in insects and fruit every night. Their diet, location, frequency of death, and what plants grow give us clues about changes in climate and planetary health.
Ice ages or glacial periods are times when glaciers cover a large areas of the surface of the earth. We know they have happened in the past from geology and fossil records, but we don’t know when the next one will be. Several factors contribute, including changes in the Earth’s tilt and orbit, atmospheric conditions, volcanism, the sun’s activity and the motion of tectonic plates. This leaves us with an incredibly complex model. While we know all these factors can contribute to an ice age era, we don’t know if all are necessary and to what degree. Developing a means of replicating and analysing the conditions would be the most conclusive way to discover which factors have the greatest effect in altering the earth’s temperature.
Some scientists even argue that there were periods in the earth's history where it was completely covered in ice – the so-called "snowball earth" – when average global temperature were -50 oC as the ice and snow reflected away a large proportion of the sun's radiation. It is thought that these periods were partially caused by a decrease in greenhouse gasses, as the activity of CO2 consuming microbes increased due to ideal conditions. As the planet cooled, the activity of these microbes decreased again, allowing greenhouse gasses to build up once more, and the snowball earth to end. But while this might have contributed to these eras, it’s not enough to explain the snowball earth entirely. It is likely a number of factors coincided to cause it, but for now how exactly the snowball Earth happened, or whether it even did, is still under debate.
A typical ice age lasts 400,000 years. These are punctuated by interglacial periods or warmer stages, which usually last about 11,500 years. Currently we are 12,000 years into an interglacial period, which could be taken as implying that another ice age is imminent, but scientists aren’t so sure…
This interglacial period might be longer because the Earth’s tilt and its orbit are about 10,000 years out of phase. The earth’s orbit isn’t a perfect circle, we’re sometimes slightly closer to the sun and sometimes further away – giving us seasons. But there’s more land in the northern hemisphere, and more water in the southern hemisphere: although land heats and cools more quickly than water, water retains more heat than land does. So if we were closer to the sun during the northern hemisphere’s summer, the whole earth is cooler.
Given that the tilt and orbit are so out of phase, their cumulative effect is far weaker, hence scientists believe this alteration in conditions might elongate our current warm phase, not to mention human-driven climate change, which adds an unpredictable variable into the mix!
Humans are producing huge amounts of CO2, which is contributing to global climate change. But what exactly happens to the CO2 that we release isn't fully understood. NASA launched the Orbiting Carbon Observatory-2 satellite in 2014 to help map and sample CO2 levels and, hopefully, to find out.
We know that there are natural carbon sinks in the world – areas that absorb more carbon dioxide than they release. Examples of these include bodies of water and plants photosynthesising. Unfortunately, little is known about how much CO2 these sinks can mop up, and whether they will get “full”. Understanding how these sinks work will help to predict the future of climate change, and may even give clues to how we could build artificial sinks to mitigate our own impacts on the environment.
There has recently been some debate over whether global warming has started slowing down after NASA conducted a study of the temperature of the deep ocean. Using satellites, information was collected from 2005 - 2013 from ocean depths below 1,995m. The data demonstrated the temperature had not increased notably over that time, despite the effects of global warming still progressing.
Greenhouse gases are still progressively accumulating in the atmosphere, but while surface temperatures and sea levels are still rising, they are not doing so at the rate believed to correspond to the the amount gases released.
So with both the surface and deep ocean not behaving as expected with respect to temperature, we are left asking - where is the missing heat going?
A number of previously unseen or changing atmospheric phenomena may be a result of the changing temperatures. One of these is the appearance of Polar Mesospheric Clouds. These "noctilucent," or night shining, clouds are so high up they are illuminated by the sun even when it is below the horizon, giving them an eerie glow. They form over both poles during the summer, and have been observed since 1885, but their brightness and frequency seems to be increasing. They are also being seen at lower latitudes. Although we don't know how or why they form, it is thought that their increase could be linked to differences in atmospheric temperature due to climate change.
What effects will climate change have?
We don't know exactly what will happen if climate change continues unchecked, but one problem that seems unavoidable is the rise in sea levels. As temperatures rise, ice will melt and the water in the sea itself will expand. This could devastate coastal populations of humans as well as animals and plants.
However, we don't yet know how big a problem rising sea levels will be, as there are so many factors to take into account. The rate the ice melts and differences in evaporation and rainfall will have an impact, as will changes in the atmosphere's ability to hold water. While scientists are attempting to model these changes the number of different factors involved, and the fact that they are all interrelated, makes this a very challenging task.
Learn more about water.
Another relevant question is “can animal species adapt to cope with a change in conditions”? Different animals react very differently to changes in their habitat, so it is impossible to predict how every species will cope. Those that are likely to be hardest hit are the ones living in areas with very little temperature fluctuation, such as the poles or the tropics. These creatures are perfectly adapted for the temperature they live in, and even small changes could prove devastating for them.
A study on evolution and heat tolerance was carried out in 2011 on a tiny sea creature, commonly found in tide pools, known as a Tigriopus Californicus. While the species illustrated a large variation in heat tolerance, due to living in different in different locations before the study, the Tigriopus Californicus from the same environment demonstrated a maximum tolerance of half a degree Celsius warmer over ten generations, with most floundering before that point.
It is believed that many species are already at their upper limit of heat tolerance, and it seems likely that in many situations evolution and natural selection will not outrun climate change. If even one species nears extinction it can unbalance the ecology completely, causing a chain reaction and affecting multiple species that were reliant in some respect on the now extinct species, whether directly or indirectly.
Heat related deaths would increase as a direct result of global warming2, but the physiology of a healthy human could handle a slight increase in temperature. Whether civilisation could cope with the changes this increase may incur to our environment however, is up for debate.
Learn more about evolution.
From an economic perspective, global warming is predicted to have a direct economic cost , although working out what that might be is a huge task. An increase in droughts and flooding due to more extreme weather conditions, along with damage caused by sea levels rising, and other problems associated with the rising temperatures are could cost $200bn for the EU alone if global temperatures increase by 3.5 degrees, as is currently expected. Heat-related deaths could also reach about 200 000 a year.
Learn more about weather.
So what can we do to prevent the possible devastation climate change could cause? One important factor is to cut our carbon emissions. Burning of fossil fuels accounts for the majority of our CO2 emissions, so finding a replacement would be a huge step forward. One possibility would be using hydrogen. Hydrogen is an effective fuel, but most methods of production are very expensive, inefficient and/or require the use of fossil fuels.
Now scientists at the University of Virginia have created a combination of enzymes which can break down plants, including waste food, to produce hydrogen. This happens at relatively low temperatures, so shouldn't be as costly as current methods.
While this is exciting research, the process still needs to be scaled up, and its efficiency maximised, before it can be brought onto the market. The safety problems of using hydrogen (which is extremely explosive) as a fuel also need to be tackled. But who knows? In the future we may be stopping by the side of the road to stock up on sugar, rather than petrol.
consequences - is this a mass extinction now?
consequences How rising temperature may effect rainfall is not known.3
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This article was written by the Things We Don’t Know editorial team, with contributions from AlmeidaJ, Ed Trollope, Jon Cheyne, Cait Percy, Johanna Blee, Grace Mason-Jarrett, Jon Tennant, Rowena Fletcher-Wood, Joshua Fleming, and Holly Godwin.
This article was first published on 2015-08-27 and was last updated on 2019-06-14.
why don’t all references have links?
 Grosberg, R.K. Kelly, M.W. Sanford, E. “Limited potential for adaptation to climate change in a broadly distributed marine crustacean” Proceedings of the Royal Society B: Biological Sciences, 2011
 Graeber, D.J. “EU wary of economic hit from climate change” UPI, 2014
 Rasmussen, C. “NASA Study Finds Earth’s Ocean Abyss Has Not Warmed” JET Propulsion Laboratory, California Institute of Technology, 2014