It’s snowy here today in Washington, D.C., so it’s a great day to list the reasons why climate scientists think the global warming — which Al Gore said would melt the polar ice caps by now — has been on hiatus for the past 17 years.

Climate scientists have been stumped by the “pause” in global warming, which virtually none of the climate models predicted. Explanations from declining solar activity to oceans absorbing excess heat to Chinese use of coal have been thrown around as reasons for why the global surface temperatures stopped warming since the late 1990s.

Here are the top eight reasons why climate scientists believe that the Earth is no longer getting warmer:

1) Declining solar activity — Some scientists and climate experts have been pointing to declining solar activity for the lack of warming during most of the last two decades. The basic idea is that solar activity reached its maximum during the late 1990s and has since declined at a very fast rate — which would explain rising global temperatures during the latter half of the 20th century and the slight global cooling since the late 1990s.

In fact, German scientists found last year that solar activity and the 65-year Atlantic and Pacific Ocean oscillation cycle account for virtually all of the global warming since 1870 — when the “Little Ice Age” ended. Now with solar activity declining and ocean oscillation in its cooling phase, we could be in for a cold century.

2) Oceans ate up all the warming — Some climate scientists have argued that the oceans have “eaten up” all of the warming that has occurred since the late 1990s, which is why global surface temperatures have not risen since then.

“The increase in the amount of heat in the oceans amounts to 17 x 1022 Joules over the last 30 years,” according to the RealClimate blog, which is written by climate scientists. “That is so much energy it is equivalent to exploding a Hiroshima bomb every second in the ocean for thirty years.”

Though one has to wonder if the ocean has absorbed so much heat, why didn’t the climate models predicted catastrophic global warming by now take this fact into account?

3) Chinese use of coal has caused global cooling — No, that’s not a typo — some scientists have argued that exploding Chinese coal use has caused the globe to cool. Though such an explanation runs counter to the narrative that burning fossil fuels warms the planet. If Chinese coal can keep the planet from heating up, why not burn more coal?

So how does this happen? Sulfur dioxide particles in the air deflect sunlight, which cools the atmosphere a bit. This happens even as coal produces carbon dioxide, which supposedly also heats up the planet.

“People normally just focus on the warming effect of CO2 (carbon dioxide), but during the Chinese economic expansion there was a huge increase in sulfur emissions,” which have a cooling effect, Robert K. Kaufmann of Boston University told the Huffington Post.

4) Montreal Protocol slowed global warming — That’s right, a government solution that actually may have worked to slow down global warming, according to researchers. Climatologist Francisco Estrada said that the pause in global warming is the unintented consequence the Montreal Protocol, which banned chlorofluorocarbons, or CFCs, in the 1980s because of their effect on the ozone layer.

Though experts warned that this is unlikely to be the only reason why global warming hiatus. There are likely many contributing factors to the pause, says Felix Pretis, a statistician at the University of Oxford.

5) There is no pause in global warming — While some climate scientists try to explain away the pause in global warming, others have denied outright that global warming has stopped at all. One paper even argues that the planet has warmed twice as fast since 1997 — around the time when the warming pause began.

Certain areas of the world — like the Arctic, Antarctica and parts of Africa — don’t have consistent climate monitoring. In other words, they are gaps in the data sets. What Kevin Cowtan from the University of York and Robert Way from the University of Ottawa did was fill in those gaps, and in doing so found that the world was warming twice as fast as in previous decades.

Though climate scientist Dr. Judith Curry warns that simply filling in missing data comes with its own perils, adding that the Cowtan and Wray study doesn’t add “anything to our understanding of the global surface temperature field and the ‘pause.’”

6) Volcanoes — Basically, the idea is that global warming has stopped because dozens of volcanoes have been spewing sulfur dioxide into the atmosphere.

“Small amounts of sulfur dioxide emissions from Earth’s surface eventually rise 12-20 miles into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulfuric acid and water particles that reflect sunlight back to space, cooling the planet,” according to a research team led by the University of Colorado Boulder.

7) Declining atmospheric water vapor — NASA data from last year showed that declining water vapor in the upper atmosphere has had a cooling effect on the planet that is 16-times greater than the warming effect from greenhouse gases created by mankind during the period of 1990 to 2001.

“Current climate models do a remarkable job on water vapor near the surface. But this is different — it’s a thin wedge of the upper atmosphere that packs a wallop from one decade to the next in a way we didn’t expect,” said Susan Solomon, senior scientist at NOAA in 2010.

That year, NOAA found that water vapor in the stratosphere had decreased by 10 percent in the last decade, causing surface temperatures to cool by 25 percent. Increasing stratospheric water vapor during the 1990s likely helped accelerate warming during that time by about 30 percent.

8) Pacific trade winds stopped global warming — This is the latest explanation for why global warming has been on pause for the last 17 years. New research argues that stronger than normal trade winds in the Pacific have driven warm water from the surface and down to the ocean’s depths, which reduces the amount of heat released into the atmosphere.

The one problem with this theory is that it directly contradicts claims made a few years ago which said that Pacific trade winds were slowing down due to global warming. Whoops!

“The vast loop of winds that drives climate and ocean behavior across the tropical Pacific has weakened by 3.5 percent since the mid-1800s, and it may weaken another 10 percent by 2100,” according to research by University Corporation for Atmospheric Research scientist Gabriel Vecchi from 2006. “The study indicates that the only plausible explanation for the slowdown is human-induced climate change.”

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