Charlie Riedel / AP

In this Sunday, Dec. 2, 2012 photo, a flock of geese fly past the smokestacks at the Jeffrey Energy Center coal power plant as the suns sets near Emmett, Kan.

WASHINGTON (AP) - The old saying that "what goes up must come down" doesn't apply to carbon dioxide pollution in the air, which just hit an unnerving milestone.

The chief greenhouse gas was measured Thursday at 400 parts per million at the Mauna Loa monitoring site in Hawaii that sets the world's benchmark. It's a symbolic mark that scientists and environmentalists have been anticipating for years.

While this week's number has garnered all sorts of attention, it is just a daily reading in the month when the chief greenhouse gas peaks in the Northern Hemisphere. It will be lower the rest of the year. This year will probably average around 396 ppm. But not for long — the trend is going up and at faster and faster rates.

Within a decade the world will never see days — even in the cleanest of places on days in the fall when greenhouse gases are at their lowest — when the carbon measurement falls below 400 ppm, said James Butler, director of global monitoring at the National Oceanic and Atmospheric Administration's Earth Science Research Lab in Boulder, Colo.

"The 400 is a reminder that our emissions are not only continuing, but they're accelerating; that's a scary thing," Butler said Saturday. "We're stuck. We're going to keep going up." 

Carbon dioxide stays in the air for a century, some of it into the thousands of years. And the world carbon dioxide pollution levels are accelerating yearly. Every second, the world's smokestacks and cars pump 2.4 million pounds of the heat-trapping gas into the air.

Carbon pollution levels that used to be normal for the 20th century are fast becoming history in the 21st century.

"It means we are essentially passing one in a whole series of points of no return," said Michael Mann, a climate scientist at Pennsylvania State University.

Princeton University climate scientist Michael Oppenheimer said the momentum in carbon dioxide emissions has the world heading toward and passing 450 ppm. That is the level which would essentially mean the world warms another 2 degrees, what scientists think of as dangerous, he said. That 2-degree mark is what much of the world's nations have set as a goal to prevent.

"The direction we've seen is for blowing through the best benchmark for what's dangerous change," Oppenheimer said.

And to see what the future is, scientists look to the past.

The last time the worldwide carbon level probably hit 400 ppm was about 2 million years ago, said Pieter Tans of the National Oceanic and Atmospheric Administration.

That was during the Pleistocene Era. "It was much warmer than it is today," Tans said. "There were forests in Greenland. Sea level was higher, between 10 and 20 meters (33 to 66 feet)."

Other scientists say it may have been 10 million years ago that Earth last encountered this much carbon dioxide in the atmosphere. The first modern humans only appeared in Africa about 200,000 years ago. 

Environmental activists, such as former Vice President Al Gore, seized on the milestone.

"This number is a reminder that for the last 150 years — and especially over the last several decades — we have been recklessly polluting the protective sheath of atmosphere that surrounds the Earth and protects the conditions that have fostered the flourishing of our civilization," Gore said in a statement. "We are altering the composition of our atmosphere at an unprecedented rate." 

Carbon dioxide traps heat just like in a greenhouse. It accounts for three-quarters of the planet's heat-trapping gases. There are others, such as methane, which has a shorter life span but traps heat more effectively. Both trigger temperatures to rise over time, scientists say, which is causing sea levels to rise and some weather patterns to change. 

When measurements of carbon dioxide were first taken in 1958, it measured 315 ppm. Some scientists and environmental groups promote 350 ppm as a safe level for CO2, but scientists acknowledge they don't really know what levels would stop the effects of global warming.

The level of carbon dioxide in the air is rising faster than in the past decades, despite international efforts by developed nations to curb it. On average the amount is growing by about 2 ppm per year. That's 100 times faster than at the end of the Ice Age.

Back then, it took 7,000 years for carbon dioxide to reach 80 ppm, Tans said. Because of the burning of fossil fuels, such as oil and coal, carbon dioxide levels have gone up by that amount in just 55 years. 

Before the Industrial Revolution, carbon dioxide levels were around 280 ppm, and they were closer to 200 during the Ice Age, which is when sea levels shrank and polar places went from green to icy. There are natural ups and downs of this greenhouse gas, which comes from volcanoes and decomposing plants and animals. But that's not what has driven current levels so high, Tans said. He said the amount should be even higher, but the world's oceans are absorbing quite a bit, keeping it out of the air. 

"What we see today is 100 percent due to human activity," said Tans, a NOAA senior scientist. The burning of fossil fuels, such as coal for electricity and oil for gasoline, has caused the overwhelming bulk of the man-made increase in carbon in the air, scientists say. 

The world sent 38.2 billion tons of carbon dioxide into the air in 2011, according international calculations published in a scientific journal in December. China spews 10 billion tons of carbon dioxide into the air per year, leading all countries, and its emissions are growing about 10 percent annually. The U.S. at No. 2 is slowly cutting emissions and is down to 5.9 billion tons per year. 

The speed of the change is the big worry, said Pennsylvania State's Mann. If carbon dioxide levels go up 100 ppm over thousands or millions of years, plants and animals can adapt. But that can't be done at the speed it is now happening. 

"We are a society that has inadvertently chosen the double-black diamond run without having learned to ski first," NASA climate scientist Gavin Schmidt said. "It will be a bumpy ride." 

Seth Borenstein can be followed at http://twitter.com/borenbears

By Douglas Main
LiveScience

Although it's still chilly throughout much of the country, spring will soon turn up the heat.

The majority of the United States will experience above-average temperatures over the next three months, said Laura Furgione, deputy director for the National Weather Service (NWS). Her remarks came during the National Oceanographic and Atmospheric Administration's spring climate outlook briefing Thursday morning.

Drought is also expected to continue or worsen in much of the Southwest and central plains, Furgione said during the briefing. About 51 percent of the country is in moderate or worse drought, she added.

Drought, flooding, warmth
"We expect it to be drier than normal in much of the West, Rockies and Southwest, including Texas, which is unfortunate since we're looking at continued drought there," Furgione said.

At the same time, flooding is likely in various regions, with the most severe possibilities occurring in North Dakota along the Red River, she said. The upper and middle Mississippi River valley could see some flooding, worsened by melting of late season snow, she said.  

"This spring's outlook is a mixed bag of drought, flooding and warm weather," Furgione said.

The spring equinox, the official start to the season, happened yesterday (Wednesday) at 7:02 a.m. EDT (11:02 a.m. Universal Time). [Season to Season: Earth's Equinoxes & Solstices]

One factor in the season's outlook is the currently neutral El Niño-La Niña climate cycle. "It's about as neutral as I've ever seen it," said Ed O'Lenicof the NWS' Climate Prediction Center.

Severe weather, tornadoes
What the climate outlook means for this spring's tornado season is hard to say.

"Neutral conditions tend to favor slightly above-normal severe weather in the Midwest and South, but that becomes weaker as we move toward April and May," added Greg Carbin, a meteorologist with the NWS' Storm Prediction Center, in a separate interview.

There has been a slightly above-average number of tornadoes so far this year, although that's not likely to continue over the next two weeks because of cold conditions — which don't favor twisters — throughout much of the Midwest and East, Carbin said.

NOAA

NOAA's GOES-13 satellite captured this image of the Earth at the spring equinox, Wednesday morning at 7:45 a.m. Eastern Daylight Time.

Beyond two weeks, however, it's very difficult to tell whether there are likely to be above- or below-average number of tornadoes. "It's not something the science is capable of giving us," Carbin told OurAmazingPlanet. 

However, it's "a given" there will be more tornadoes in April and May, because that's when there are more tornadoes on average, he said.

But the past is a poor indicator when it comes to tornadoes. Last year the tornado season was unusually active early in the year, before trailing off through the spring and summer; by year's end, there were fewer tornadoes than ever on record, Carbin said. This stood in contrast to the previous spring, which was a record-setting year for tornadoes and the deaths and damage they caused.

The continuing drought and likelihood of increasing temperatures could create fire hazards in the Southwest. Oklahoma has already seen wildfires break out, Furgione said. Dry conditions in Florida also have led to fires, according to the National Interagency Fire Center.

Email Douglas Main or follow him @Douglas_Main. Follow us @OAPlanet, Facebook or  Google+. This article originally appeared on LiveScience's OurAmazingPlanet.

• 6 Signs That Spring Has Sprung
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• Mightiest Floods of the Mississippi River

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NOAA

The GOES-13 satellite captured this full-disk image of our planet at 7:45 a.m. ET on March 20, just after the 7:02 a.m. ET equinox. The satellite image shows how Earth's two hemispheres receive equal amounts of sunlight during the equinox. In this image, the sun is artificially created to enhance the picture.

Earth's 23.5-degree tilt almost always ensures that the northern and the southern halves of our planet get unequal amounts of solar energy, with longer nights in winter and bigger stretches of sunlight in summer. Twice a year, however, both hemispheres get equal amounts of light, with equal intervals of day and night. That's what's known as the equinox.

Just such an event at 7:02 a.m. ET on Wednesday heralded the official beginning of spring in the Northern Hemisphere, and the start of autumn in the South. This full-disk picture from the GOES-13 weather satellite, captured at 7:45 a.m., shows the equal division between Earth's night and day.

"The visible imagery sensor on GOES requires sunlight to 'see' clouds, and so it provides a useful example of the equinox," the National Oceanic and Atmospheric Administration's Environmental Visualization Laboratory says in Wednesday's advisory. "In this image the GOES imagery extends to each of the poles since the entire hemisphere is equally lit. After the equinox passes today, the Northern Hemisphere will be more lit than the Southern Hemisphere – causing the seasons."

Orbital mechanics may determine the precise moment of the equinox, but scientists say that the effects of the seasonal change can vary widely, due to climatic factors. There's some evidence, for example, that climate change is causing flowers to bloom earlier in the eastern U.S. than they did in the 1850s or the 1930s. Have you noticed changes on shorter time scales? Feel free to spring into action with your comments below.

More about the changing seasons:

• How we know that spring has sprung
• Spring begins a day earlier, kind of
• Gallery: 10 spring flings with science

Tip o' the Log to LiveScience's Douglas Main.

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log and the rest of NBCNews.com's science and space coverage, sign up for the Tech & Science newsletter, delivered to your email in-box. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

NASA Earth Observatory

NASA's Earth Observatory website is conducting a "March Madness" tournament that pits the 32 best Earth images from 2012 against each other.

By Megan Gannon
LiveScience

March Madness is coming, and if you care more about solar flares than foul shots or would sooner bet on a real cardinal than the Louisville Cardinals, then you have some alternative brackets to take part in this month.

Bird-lover bracket
"March Madness is happening just when all the birds are starting to return from Central and South America, and it's a really exciting time for a bird watcher," said Hugh Powell, a science editor at the Cornell Lab of Ornithology in Ithaca, N.Y., which is holding its third annual Migratory Madness bracket.

The tournament was born out of lab staff members' conversations about picking their favorite birds, Powell told OurAmazingPlanet. (It's sort of like the endless conversations kids can have around questions like, "If Superman and Batman got into a fight, who would win?" he said.)

The 16 feathered-contenders won't be announced until Monday, and the eventual winner will have to survive four rounds of public votes. The entrants were chosen largely through polls on the Facebook pages of Cornell's various citizen science projects, including Project FeederWatch, which encourages people to send in tallies of the birds they spot at their feeders during the winter months.

Powell said there might be some surprises thrown into the mix this year (maybe even some mammals chosen by Cornell's other labs), and there should be a couple of endangered species, including the whooping crane (Grus americana), North America's tallest bird.

Powell said more common birds typically get a lot of votes, but the tournament has seen some "incredible upsets." Last year, a surprising spike in snowy owl sightings across America sent the stately bird into the top spot, and in 2011 the diminutive black-capped chickadee edged out heavyweights like the red-tailed hawk.

Once the contest is live, you will find a link to the bracket on the lab's Facebook page.

Sea creature face-off
Meanwhile, leatherback turtles, humpback whales, sawfish and sturgeon are facing off in a bracket launched by the U.S. National Oceanic and Atmospheric Administration's Fisheries Service.

This year marks the 40th anniversary of the Endangered Species Act, which protects the nation's plants and animals that are threatened with extinction, and NOAA Fisheries is commemorating the law with its tournament of 16 endangered sea-dwelling creatures.

Round 1 of the bracket runs only until Sunday; you can cast votes to decide which of your favorite competitors — Blue whale or Guadalupe fur seal? Right whale or olive ridley turtle? —gets to move on.

The list also includes some less charismatic species, like the bottom-feeding gulf sturgeon, Johnson's seagrass and the white abalone, a tough-shelled large sea snail found off the west coast of Mexico and California. NOAA's site quotes the abalone: "I may look like a 6-inch-long oval shell with a single orange foot poking out, but there's a lot more to my story than my good looks. I'm actually the first marine invertebrate to make it onto the Endangered Species Act. Before me, everyone on the list had a spine!"

Battlefield Earth
NASA Earth Observatory is also getting on board with March Madness through its own tournament of remote-sensing science.

"They are the best Earth images of the year, the top 32 from 2012. But which ones will be good enough to survive head-to-head competition?" the Earth Observatory asked on its site.

The field of two-dimensional competitors has been split up into four categories: Earth at night, events, data and true-color. Rivalries include satellite shots of Hurricane Isaac in the Gulf and Hurricane Sandy covering much of the Northeast, as well as images of New Zealand's Mount Tongariro erupting and a new volcanic island forming in the Red Sea.

The entrants also include an image of the sun during an intense January 2012 solar flare, a Black Marble nighttime view of the entire planet and a picture snapped by an astronaut aboard the International Space Station showing an elusive red sprite, an ultrafast burst of electricity that hovers for just a few milliseconds above thunderstorms.

Voting will take place each week, with the first round ending Friday (March 8) at 4:00 p.m. ET.

Follow OurAmazingPlanet @OAPlanet, Facebook or Google+.

• Image Gallery: Earth As Art
• Earth Pictures: Iconic Images of Earth from Space
• Bigger Is Better: 10 Huge Images of the Planet

Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Hadfield / Canadian Space Agency via @Cmdr_Hadfield

Canadian astronaut Chris Hadfield snapped this photo of Earth from space from the International Space Station during the Expedition 34 mission. The January 2013 photo shows Newfoundland and Labrador from orbit.

By Jeanna Bryner
LiveScience

As a blizzard makes its way toward the Northeast, numbers suggest the world is actually pretty hot, with temperatures across the globe making last month the second-warmest January in the past 35 years.

Globally, January had an average temperature that was 0.92 degrees Fahrenheit (0.51 degrees Celsius) above a 30-year baseline average, said John Christy, a professor of atmospheric science and director of the Earth System Science Center at The University of Alabama in Huntsville.

This January takes a back seat only to January 2010, with January 1998 coming in as the third warmest during this period.

During the past month, the globe's coldest spot centered on Russia, near the town of Nyagan, where temperatures averaged about 4.5 degrees F (2.51 degrees C) below seasonal norms there. Meanwhile, as compared with seasonal norms, the Norwegian Arctic archipelago of Svalbard (located north of Norway and east of Greenland) showed the warmest temps in January. There, the thermometer rose some 7.4 degrees F (4.1 degrees C) above January norms.

The global numbers are part of an ongoing project between the University of Alabama, the National Oceanic and Atmospheric Administration (NOAA) and NASA. Instruments aboard NOAA and NASA satellites measure the temperature of the atmosphere from Earth's surface to an altitude of about 5 miles (8 kilometers) above sea level. Once processed, the monthly temperature data is placed in a publicly available computer file.

January heat follows right on the heels of the warmest year on recordin the contiguous United States, stretching back to the 1880s, a record that NOAA scientists announced last month.

Every contiguous U.S. state had an above-average annual temperature for 2012, with 19 states boasting a record warm yearand an additional 26 states experiencing one of their 10 warmest years, NOAA's National Climate Data Center (NCDC) reported.

As for what's behind the warming trends, "It's a combination of longer-term trends and local effects or regional effects like the drought," NCDC climatologist Jake Crouch told LiveScience in November, referring to the widespread drought conditions in the United States in 2012.

Follow LiveScience on Twitter @livescience. We're also on Facebook  and  Google+.

• The Reality of Climate Change: 10 Myths Busted
• Photos: The Coldest Places on Earth
• Image Gallery: One-of-a-Kind Places on Earth

Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

If you found yourself bundling up in scarves, hats, and long underwear less than usual last year, you weren't alone: 2012 was the warmest year on record in the contiguous United States, according to scientists with The National Oceanic and Atmospheric Administration.

The average temperature for 2012 was 55.3 degrees Fahrenheit, 3.2 degrees above normal and a full degree higher than the previous warmest year recorded -- 1998 -- NOAA said in its report Tuesday. All 48 states in the contiguous U.S. had above-average annual temperatures last year, including 19 that broke annual records, from Connecticut through Utah.

“We’re taking quite a large step,” said Jake Crouch, a climate scientist from the NOAA National Climatic Data Center, which has recorded temperatures in the contiguous U.S. for the past 118 years.

It was also a historic year for "extreme" weather, scientists with the federal agency said. With 11 disasters that surpassed $1 billion in losses, including Superstorm Sandy, Hurricane Isaac, and tornadoes across the Great Plains, Texas, and the Southeast and Ohio Valley, NOAA said 2012 was second only to 1998 in the agency's "extreme" weather index.

A long-term warming trend for the U.S., combined with drought and a northerly jet stream, led to the record heat, explained Crouch. 

"During the winter season, the jet stream tended to stay further north of the U.S.-Canadian border, so that limited colder outbreaks in the country. It also limited precipitation. So that led to a warm and dry winter season, and that persisted through the spring," he said. 

Matt Rourke / AP file

People play in water from an open fire hydrant during the afternoon heat on July 18, 2012, in Philadelphia. July was the hottest month ever on record in the contiguous U.S.

"That warm and dry spring and winter laid the groundwork for the drought we had this summer... . When we have drought, it tends to drive daytime temperatures upward."

The unprecedented warm weather wasn't contained to the United States.

A corresponding rise in global temperatures prompted the World Meteorological Organization to call the rate at which the Arctic sea ice was melting "alarming" in its Nov. 28, 2012, report.

“The extent of Arctic sea ice reached a new record low. The alarming rate of its melt this year highlighted the far-reaching changes taking place on Earth’s oceans and biosphere. Climate change is taking place before our eyes and will continue to do so as a result of the concentrations of greenhouse gases in the atmosphere, which have risen constantly and again reached new records,” World Meteorological Organization Secretary-General Michel Jarraud said.

Each year since 2001 has been among the warmest on record worldwide, with 2012 likely to "be no exception despite the cooling influence of La Niña early in the year," the report added.

'Horrible' sea level rise of more than 3 feet plausible by 2100, experts say 

Watch NBC's special coverage of the 2012 drought 

'Wake-up call': Chicago set to break 73-year-old snowless record

NOAA expects to have global data for 2012 sometime in the coming weeks, but Crouch said scientists already know with certainty "it's going to be in the top ten" warmest years ever.

Adding to the extremes: 2012 was the driest year on record for the U.S., with 26.57 inches of average precipitation -- 2.57 inches below average. Those dry conditions created an ideal environment for wildfires in the West, which charred 9.2 million acres -- the third highest amount ever recorded, NOAA said Tuesday.

Other notable climate activity from 2012:

• Snowpack totals across the Central and Southern Rockies were less than half normal.
• July was the hottest month ever on record in the contiguous U.S.
• Tornado activity was concentrated toward the beginning of the season, with large outbreaks in March and April in the Ohio Valley and Central Plains, but the final 2012 tornado count will likely be less than 1,000 -- the least since 2002. "The factors behind that are kind of related to what was going on with the drought. We didn't have these large storm systems moving through the country, so that limited precipitation, and that also limited severe weather outbreaks," Crouch said. What made this year so high on the extreme weather index were cyclones, hurricanes, and the heat, he said.
• Alaska was cooler and slightly wetter than average, and had a record-cold January. "Their January temperatures were 14 degrees below average. Many locations in Alaska had temperatures 30 degrees below zero," Crouch said, adding that Anchorage, Alaska, set a new snow record.
• Hawaii experienced growing drought conditions, with 47.4 percent of the state experiencing moderate-to-exceptional drought at the beginning of 2012 and 63.3 percent at the end of the year. Alaska and Hawaii were not included in the bulk of NOAA's 2012 report because of terrain issues, and because scientists don't have records dating back as far as states in the contiguous U.S.

While NOAA made no meteorological forecasts for 2013, Crouch said the drought was going to continue to be an issue.

"The drought got a lot of attention this summer when it was having impacts on agriculture. More than 60 percent of the country is still in drought," he said. "And if things don't change, the drought is going to continue to be a big story in 2013."

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NOAA

Two different perspectives of the seeps, made by bouncing sound waves off rising plumes of gas.

By Douglas Main
Our Amazing Planet

A research cruise has discovered 40 previously unknown gas seeps on the seafloor off the U.S. East Coast. The plumes of gas are almost certainly methane, also known as natural gas, according to government scientists.

Methane is a potent greenhouse gas due to its ability to absorb heat, but the released gas is not likely to reach the ocean surface in significant quantities and affect the climate, said Carolyn Ruppel, a researcher with the U.S. Geological Survey, which collaborated in the research. Neither is the amount of gas likely to warrant commercial interest, she said.

The seeps were found in four clusters, three of them about 100 miles (160 kilometers) southeast of Nantucket, Mass. The other cluster, consisting of 17 of the seeps, was mapped about 90 miles (147 kilometers) east of Cape Henry, Va., according to a release from the National Oceanographic and Atmospheric Administration, which led the expedition.

Methane seeps are important to find and study since they involve the transfer of carbon from the ground to the atmosphere, which is important for getting an accurate picture of climate change in terms of how much gas is emitted naturally and how much is emitted by humans, Ruppel told OurAmazingPlanet. Methane also can oxidize in water and contribute to ocean acidification, she said. [ Video: Humans Hit the Oceans Hard ]

The NOAA ship Okeanos Explorer mapped the locations between Nov. 2 and Nov. 20 using multibeam sonar, which produces detailed images of the seafloor by calculating the amount of time and distance it takes for sound waves to travel from the ship to the seafloor and back. During that time the ship and its instruments mapped 5,970 square miles (15,460 square kilometers) of seafloor, an area larger than Connecticut, according to the NOAA release. The mapping was primarily done along the continental slope, where the North American continent ends and drops into the Atlantic Ocean basin. Sound waves were also used to visualize the rising plumes of gas.

It's unclear exactly where the gas is coming from, Ruppel said. Methane either can arise from microbial activity in shallow deposits of organic material, or it can come from more deep-seated processes involving oil formation. Probably both processes are at work in these different seeps, she said.

A mere generation ago, methane seeps were virtually unheard of off the East Coast. Since the early 1980s, however, several seeps have been found. "With advanced multibeam sonar, it may become routine to discover seeps while we systematically explore our poorly known ocean," NOAA scientist Stephen Hammond said in the statement.

Reach Douglas Main at dmain@techmedianetwork.com. Follow him on Twitter @Douglas_Main. Follow OurAmazingPlanet on Twitter @OAPlanet. We're also on Facebook and Google+.

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