Climate Change: The Details

1) The Planet is Getting Warmer

There is no doubt that our planet is getting warmer. Although there are always year-to-year changes in temperature driven by natural variation, the planet has been warming since the 1800's and has been on a steady upward trend since the 1960's. There are at least 10 separate lines of evidence that support this claim. Land and sea surface measurements show consistent increase and other responses show consistent affects, like accelerated melting of glaciers and reductions in snow cover.

adapted from US National Climate Assessment (2013)

10 Key Indicators of a Warming Planet:

1. Increased land temperatures
2. Rising sea levels
3. Increased ocean temperatures
4. Reduced sea ice in polar regions
5. Reduced snow cover in spring

6. Rapid melting of glaciers
7. Increased ocean heat content
8. Increased temperatures over oceans
9. Increased water vapor in the atmosphere
10. Increasing temperatures in troposphere

Observed warming is not confined to the Earth’s surface. Measurements by weather balloons and satellites consistently show that the temperature of the troposphere, the lowest layer of the atmosphere, has increased. The upper ocean has warmed, and more than 90% of the energy absorbed by the climate system since the 1960s has been stored in the oceans. As the oceans warm, seawater expands, causing sea levels to rise.

Warmer air will, on average, contain a greater quantity of water vapor. Globally, analyses show that the amount of water vapor in the atmosphere has also increased over the land and the oceans.

About 90% of the glaciers and land-based ice sheets worldwide are melting as the Earth warms, adding further to the sea level rise.

Spring snow cover has decreased across the Northern Hemisphere since the 1950s and there have been substantial losses in sea ice in the Arctic Ocean.

All of these indicators and the independent data sets that have been assembled point to the same conclusion: from the ocean depths to the top of the troposphere, the world has warmed. And when you consider additional data: like records from ice cores, tree rings, soil boreholes, and other forms of “proxy” climate data; they show the warming is unusually rapid.

On its own, after the Ice Age and other such cooling periods, the Earth's temperatures rise 7°F to 13°F higher over thousands of years. Currently, the rate of warming is about 8 times more rapid than what we would expect from natural factors. Yikes!

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2) Our Use of Energy from Fossil Fuels is the Cause of Climate Change

The Earth has experienced many large climate changes in the past. However, the current changes are unusual for two reasons: first, they are occurring faster than they did in the past, and second, they are primarily driven by human activities.

One metric Ton of CO2

1 metric ton = 2,204 lbs

In the past, climate change was driven exclusively by natural causes including volcanic eruptions (that throw soot and particles into the upper atmosphere), changes in energy radiated from the Sun, and periodic variations in the Earth’s orbit. Climate change is also driven by natural cycles that transfer heat between the ocean and the atmosphere, as well as slowly changing variations in heat-trapping (greenhouse), gases that alter global temperature over periods ranging from months, and in the case of volcanic eruptions, to thousands of years.

But since the beginning of the Industrial Revolution, humans have been adding greenhouse gases to the atmosphere at a much faster rate than can occur naturally; and the source is the combustion of fossil fuels.

The molecules that make up fossil fuels (coal, oil and natural gas), contain significant potential energy that is released by burning. A major by-product of this burning is carbon dioxide (CO2) gas, the same clear gas that fizzes your Coke and Pepsi. And this gas, which is emitted from car exhaust pipes and power plant smokestacks, mixes with the air in the atmosphere where it then absorbs, and re-emits infrared radiation back to the Earth's land surfaces, the oceans, and even the sky. Normally, some of the radiation would be radiated out into space, cooling the planet as part of the normal energy balance. Due to the additional CO2 in the atmosphere, however, this radiation is reflected back to warm land masses and oceans.

This process, where atmospheric gases absorb and then re-emit infrared (or thermal) radiation, ultimately leading to warming, is called the greenhouse effect. The scientific basis dates back to 1824 when the French scientist Joseph Fourier established the existence of the greenhouse effect. The heat-trapping abilities of greenhouse gases were then corroborated by Irish scientist John Tyndall with experiments beginning in 1859. Svante Arrhenius, a Swedish chemist, was the first to calculate the effect of increasing fossil fuel use on global temperature in 1890.

Carbon emissions from burning coal, oil, natural gas and cement manufacture - in units of millions of metric tons of CO2 equivalents. Adapted from US National Climate Assessment (2013)

The discovery of inexpensive fossil fuels was a boon to the Industrial Revolution. They comprise the bulk of all energy use by humans today (about 80% in 2008), providing energy on a daily basis to billions of people; and we have been pumping, mining and extracting them from the Earth about as fast as we can to meet the ever growing demand for energy. And due to the chemistry of their combustion, we have also been dramatically increasing the concentration, measured in parts per million (ppm), of C02 that is present in the atmosphere.

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3) If We Don’t Reduce Emissions Soon, Global Temperatures Could Exceed Values Consistent with Life as We Have Come to Know It.

Say what?

If we continue on the current path of fossil fuel use (i.e. "business-as-usual"), it's likely that CO2 concentrations will reach 450 ppm by 2030 and 700 ppm by the end of the century. The resulting climate changes from such high levels are forecast to be extreme.

Global temperatures would likely increase 4°C (+7.2°F), relative to pre-industrial times, reaching levels that haven't been seen for more than 800,000 years. Melting of the Greenland and the Antarctic ice sheets, and ocean expansion from heating, could lead to a sea-level rise of 0.5 to 1 meters (1.6 to 3.2 feet), by 2100. And continuing with this pattern, there would be several meters more to be realized in the coming centuries. We could expect major impacts on world agriculture, extreme weather events and more. Global temperatures could exceed values consistent with life on the planet as we have come to know it.

A 4°C increase would likely become the dominant driver of ecosystem shifts, surpassing habitat destruction as the greatest threat to biodiversity. Recent research suggests that large-scale loss of biodiversity is likely to occur with high CO2 concentrations driving a transition of the Earth´s ecosystems into a state unknown in human experience. Ecosystem damage would be expected to dramatically impact fisheries as well as the protection of coastlines by coral reefs and mangroves.

Adapted from: Turn Down the Heat, Why a 4ºC World Must be Avoided, World Bank, 2013

Given the creation of a 4°C world is untenable, and impacts on ecosystems are likely at 1 to 2°C, more than 100 countries have adopted a global warming limit of 2°C (3.6°F) or below as a guiding principle for mitigation efforts to reduce climate change risks, impacts and damages. The 2°C threshold establishes a so-called "red-line" to guide policy making, but there are some that suggest that even this level is too high. We have already warmed the planet 0.8°C (1.4°F) above pre-industrial levels.

Limiting warming to 2°C could potentially reduce sea-level rise by about 20 cm (7.8 inches) by 2100. However, global mean sea level could continue to rise with some estimates ranging between 1.5 (5 ft) and 4 (13 ft) meters above present-day levels by the year 2300. Sea-level rise would likely be limited to below 2 meters only if warming were kept below 1.5°C.

Although forecasting is an imperfect business, what's clear is the risks associated with staying on our current path are very high. If we continue with "business as usual" from an emissions perspective, we will cross the 2ºC line and risk increasing average global temperatures above the predicted 4ºC change. If we remain on this path, we will be creating a climate that has never been seen before in human history. Expert opinion is that this climate will be much tougher than it is now.

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4) We Need to Reduce C02 Emissions If We Want to Secure Our Future

Given the issue is growth of emissions, it's reasonable to think that if we just held them constant (let's say at 400 ppm), we'd have the problem licked. Unfortunately, the physics say otherwise. It turns out that, counter-intuitively, holding emissions constant leads to INCREASING global temperatures!

This remarkable fact is due to something called the lag effect. The lag effect has two primary causes. The first is that once CO2 is emitted into the atmosphere it hangs around for literally hundreds of years doing its greenhouse heating. And the second is because the oceans are so large and deep, it takes a long time for them to heat up.

It takes a century for roughly half the CO2 emitted at any one time to be removed from the atmosphere by plants and the oceans. Even worse, around 20% of the released emissions continue to circulate for thousands of years, all the while doing its greenhouse heating. What this means is that stabilizing or reducing atmospheric carbon dioxide concentrations requires very deep reductions in future emissions. This is partly because we can't add more to the CO2 (emitted by our historical energy-using activities), to those still circulating in the Earth's system.

The additional radiant heat that is re-emitted back to the Earth by the greenhouse gases heats the oceans. However, because of mixing in the top 100 meters (about the length of a football field), and because there is a whole lot of water out there, it takes a while to heat up. But also note the same thermal inertia that made the oceans slow to heat up, also make it slow to cool down. From a heating perspective, the oceans are kind of like a supertanker, tough to get moving, but also very slow to turn once they get going.

So you can start to see why even small additions of CO2 to what's up in the atmosphere already could be problematic. First, the gases that we have already emitted are doing their heating thing, and second, any additional gases are just adding to the mix. In fact, the lag effect is the primary reason we need to act immediately to reduce our energy use from fossil fuels.

To assure we don't cross the red line, we need to start reducing significantly...today. If we start now, we can reduce emissions at a manageable level (to be technical around 3.5% per year). If we delay five or ten years, it gets more and more difficult. If we don't start for another decade, it's unlikely we'll make it at all.

Adapted from: US Climate Assessment - 2013

Another, perhaps easier, way to think about emissions is to picture it like having a remaining budget.

Because of the lag effect, it turns out that the final temperature increase of the planet is directly proportional to the amount of greenhouse gas we emit from this point forward. For every additional Ton of CO2 we emit there is a permanent increase to the final (average) temperature of the planet.

Now it will take some time to reach the final temperature, the gases and oceans and entire system need to equilibrate. But once there, the planet will remain at this temperature for a long time until the excess C02 is reabsorbed by plants, the oceans and rocks/soils (which takes many centuries). And because we agree, in principle, that we can't exceed 2ºC, we can estimate the maximum amount of CO2 we can emit before we cross this red line.

Current estimates put this cap (or budget) at around 3700Pg CO2. This would give us even odds of not crossing the 2ºC line. Now this sounds like a lot, but in reality it's not. Given we've emitted half of this amount to date (we've been doing lots of work and play the last 100 years), at our current emissions rates we'd hit this in 50 years! After that our carbon budget is spent and we could use no more. It's like (fossil fuel) lights out for 9+ billion people on the planet (or more). But of course, that simply isn't going to happen.

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5) Good News! There's Still Time to Make a Difference....

Adapted from: US Climate Assessment - 2013

Despite the fact that CO2 is the gift-that-keeps-on-giving (from a greenhouse-heating perspective), the very good news is there's still time to make a difference. Through sustained efforts in energy conservation, economic incentives and investments clean energy technology, we can reduce our collective carbon footprint to stay below 2ºC in average warming for the planet .

Studies suggest that if we start reducing our energy consumption now by as little as 3 to 4% on an annual basis, we can transition to a less carbon-intensive economy and reach the permanent required reductions - all while living in a progressively cleaner world. Economic analyses suggest that incremental investment of as little as 1 to 2% of GDP per year would do the trick.

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6) But....We Need to Get Moving!

Due to the lag effect, if we delay we'll have to make steeper reductions in the future AND reduce emissions to lower levels. If we consume our emissions budget earlier, by delaying any start of reductions, we will have less to use in the future - requiring steeper reductions in emissions if we are to live within its limits.

Adapted from: US Cimate Assessment - 2013

In one hypothetical example, which uses the U.S. contribution to global reductions, delaying the start of emissions reductions for ten years (from 2010 to 2020), would result in a doubling in the annual reduction rate from 4% per year to 8% per year. This would increase the required reductions from 78% to 90% by the year 2050.

Needless to say, cutting emissions at a rate of 8% per year, for 30 years straight is a tough thing to do. Cutting rates at 4% per year over 40 years seems a lot more feasible.

Another reason we need to get moving is we're currently tracking on the high side of historical emissions forecasts. One way of looking at this is to compare current and historical emissions with some of the benchmarks established for the International Panel of Climate Change (IPCC) in 2008. These benchmarks are called "Representative Concentration Pathways (RPCs)" and represent four different possible paths for increases in CO2 concentration.

Without getting too deep into the details, these benchmarks allow scientists to compare different emissions scenarios, regardless of different assumptions about forecast mitigation and socioeconomic predictions. The pathways were first established in 2008 and are being used for the upcoming assessment sometime in early 2014. Pathway RCP8.5 represents conditions where emissions remain high (sometimes called the "business as usual pathway"), while RCP3-PD is a pathway that, if achieved through effective mitigation, will keep the planet below the 2ºC red line.

Despite pledges from the international community (including the US), global emissions are currently tracking on the highest RCP pathway. And although it's still too soon to say "too-late" for achievement of stabilization at either 1.5ºC or 2.0ºC...the door, or window of opportunity, is certainly closing.

Adapted from Peters et al, Nature Climate Change (2013)

The bottom line is we need to get moving. We're tracking on the high side of forecasts and delaying every year will make it harder in the future.

The good news is there is still time to act . The not so good news is there is a limited window of opportunity, and if we don't get moving, it could be too late.

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Still Doubting? Looking for More Detail?

There is a lot of amazing science happening today in the climate change field. Some of the references we used follow.

Also, be sure to check out our Frequently Asked Questions page and links to Consensus Reports

References

Anderegg, W. R., J. W. Prall, J. Harold and S. H. Schneider (2010). "Expert Credibility in Climate Change." Proc Natl Acad Sci U S A 107(27): 12107-12109.

US National Climate Assessment Development Advisory Committee -NCADAC- (2013). Climate Assessment Report - Draft.

Meinshausen, M., N. Meinshausen, W. Hare, S. C. Raper, K. Frieler, R. Knutti, D. J. Frame and M. R. Allen (2009). "Greenhouse-Gas Emission Targets for Limiting Global Warming to 2 Degrees C." Nature 458(7242): 1158-1162.

Hare, W. L., W. Cramer, M. Schaeffer, A. Battaglini and C. C. Jaeger (2011). "Climate Hotspots: Key Vulnerable Regions, Climate Change and Limits to Warming." Regional Environmental Change 11(1): 1-13.

International Bank for Reconstruction and Development, The World Bank. (2012). "4C Turn Down the Heat: Why a 4C Warmer World Must be Avoided."

IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY

Peters, G. P., R. M. Andrew, T. Boden, J. G. Canadell, P. Ciais, C. Le Quere, G. Marland, M. R. Raupach and C. Wilson (2013). "The Challenge to Keep Global Warming Below 2ºC." Nature Clim. Change 3(1): 4-6