The sun reflects on downtown skyscrapers as it sets through the Los Angeles smog and haze in this October 22, 2006 file photo. California is set to unveil a new weapon in its fight against global climate change November 14, 2012 when it holds its first sale of carbon emissions permits - a landmark experiment that it hopes will serve as a model for other U.S. states and the federal government. REUTERS/Lucy Nicholson/Files

When it comes to combating the heat-trapping greenhouse-gas emissions that are warming the globe, the United States has had something to cheer about lately. U.S. emissions of greenhouse gases have fallen enough that the country is more than halfway to meeting its 2020 greenhouse-gas emissions pledge.

But here's the bad news: Even if the United States keeps up its trend (though it could soon hit some bumps), it's barely putting a dent in global greenhouse-gas emissions. And those global emissions have helped the Earth hit some new milestones recently for the amount of these gases in the atmosphere.

This new video from the Cooperative Institute for Research in Environmental Sciences and NOAA's Earth System Research Laboratory not only shows the recent global carbon-dioxide trend, but it also shows that humans have increased carbon-dioxide levels rapidly, and to their highest levels in at least 800,000 years:

Our latest dubious carbon milestone

Last year, for the first time in recorded history, the atmospheric level of carbon dioxide — the primary greenhouse gas that humans are emitting, mainly by burning fossil fuels — hit 400 parts per million. That may not seem like much, and 400 ppm doesn't represent a tipping point for our climate, but it's still nearly 50 percent higher than it was before industrial times began.

Last year's 400-ppm breach was actually short-lived because it occurred in the spring at the peak of our planet's seasonal carbon-dioxide cycle. (Earth's plant matter is concentrated in the northern hemisphere, so global carbon-dioxide values peak in the spring, after which the summertime plant and leaf growth up north sucks out some of the carbon dioxide.) Once fall and winter come around, this trend reverses, and some carbon dioxide is let back out.

Though that 400-ppm milestone was temporary last year, it’s a sign of things to come. This year, we hit 400 ppm two months earlier than last year. Not only that, but we've also had our first full month above 400 ppm in recorded history.

One of the carbon-dioxide emissions curves shown in the video is particularly famous. Labeled "Mauna Loa" here, it's known as the Keeling curve, and it draws on measurements taken from the Mauna Loa Observatory in Hawaii. You’ll see the seasonal carbon-dioxide cycle is especially marked in that curve, though the long-term upward trend wins out in the end. Think of the seasonal cycle, superimposed on the long-term trend, as being like a hill full of pine trees. Each tree's pointy shape represents the seasonal rise and fall of carbon dioxide. But each subsequent tree reaches slightly higher into the sky than the tree just below it, thanks to the upward slope of the hill. The carbon-dioxide level in the air should fall below 400 ppm again later this year as summer ramps up, but we don't have many years left until we eclipse 400 ppm for good.

How much progress we still have to make on carbon-dioxide levels 

One thing to keep in mind is that 400 ppm is mainly a mathematical and psychological milestone. We probably haven't hit any major tipping points by hitting 400 ppm. But the milestone still shows how close we are to hitting a more significant target from a policymaking and climatological standpoint. The United Nations has long said that the world must keep warming below 2 degrees Celsius to prevent catastrophic effects, and to do that we’ll likely need to keep carbon-dioxide levels below 450 ppm (this threshold isn't exact). Note that this 450-ppm value actually includes both carbon-dioxide and carbon-dioxide-equivalent amounts of other greenhouse gases (such as methane and nitrous oxide), but other factors such as aerosol pollution are generally canceling out the effects of the non-carbon-dioxide gases, at least for now.

Given the greenhouse-gas trajectory we’re on, the U.N.'s Intergovernmental Panel on Climate Change recently said the world would need to cut its carbon-dioxide-equivalent emissions by 40 to 70 percent by mid-century, and zero out these emissions by the end of the century. For comparison's sake, if we don't do anything more to cut carbon-dioxide emissions, we may reach 450 ppm by 2030.

Hitting the IPCC's emissions targets won't be an easy task, as Brad Plumer noted recently, and progress internationally to secure emissions cuts has been wanting. The longer we wait, the more it will cost the economy in a given time period to meet these targets. But as the IPCC warns, it could also be very costly to do nothing.