Christopher Henshilwood / University of the Witwatersrand

Bifacial points recovered from Blombos Cave, South Africa. The tools were manufactured during the Middle Stone Age by anatomically modern humans and are made of silcrete and finished by pressure flaking. Scale bar = 1cm

Sophisticated stone tool-making, artistic symbolism and trade networks were all innovated during times in the Stone Age when the South African climate abruptly became warmer and wetter, according to a new study.

The research is the first to "show that there is a link between the occurrence of these cultural innovations and climate change," study leader Martin Ziegler, an earth science researcher at Cardiff University in Wales, told NBC News.

South Africa got warmer and wetter as the Northern Hemisphere became cold and dry during periodic Ice Age slowdowns in an ocean circulation that brought warm water from the tropics north, he added. 

This allowed warm and wet conditions to prevail in South Africa for centuries to thousands of years at a time between 100,000 and 40,000 years ago, according to the study published Tuesday in Nature Communications. 

The findings are based on analysis of marine sediments dumped into the ocean from rivers flowing off of South Africa, which Ziegler and colleagues used to reconstruct climate variability over the past 100,000 years. 

"There is a very good fit between rapid climate change and the occurrence and disappearance of these first evidences of modern behavior in early humans," he said.

Abundance breeds innovation
Humans need water. Plants need water. So too do the animals that humans hunt and eat. These conditions thus are favorable for population growth, explained Chris Stringer, an authority on human origins at London's Natural History Museum, and a study co-author.

Modeling research from other scientists, Stringer noted, suggests that as human population density increases, people are able to network more readily, share ideas and invent technologies. The new findings, he said, fits with the idea that population density breeds cultural innovation.

"Those dense populations are forming networks over the landscape which is no longer huge patches of arid land that they cannot cross," he told NBC News. "They are connecting with other populations and lo and behold … we get these cultural innovations."

Innovations from the time include "an explosion of what seems to be symbolic behavior," Stringer noted, such as messages written in ochre, a type of pigment, and seashell jewelry perhaps used to establish social rank.

Sophisticated stone tools with adhesives that require complex processing of materials gathered over a broad area suggest trade networks existed.

"We find that stone tools raw materials are traveling sometimes hundreds of kilometers [making] it likely that there are trading networks between different groups passing these materials backwards and forwards," Stringer said.

The findings "support our view, which is that it is population density that is really driving innovation and connectedness," Mark Thomas, a geneticist at University College London, who led the earlier modeling work but was not involved with the new research, told NBC News.

Other proposed drivers for cultural innovation include genetic mutations that re-wired the human brain, and necessity driven by worsening environmental conditions, he noted.

"We say necessity is the mother of invention," Thomas said. "I'm not sure it is. I think the first response to necessity if you haven't got the invention is dying."

Lessons from the past
According to Ziegler, the finding of this link between climate and cultural change adds to a growing list of studies that indicate cultures from human ancestors to the Maya have been affected by shifts in the climate.

"So it is another hint for us that we should keep an eye on the climate because when it is changing abruptly and largely in the past it has always affected humans and so it may do so in the future as well," he said.

Stringer added that the human population today linked by global trade, social networks such as Facebook, and rapidly evolving technologies such as mobile phones are the fruits of a climate that has been relatively stable for 11,000 years. 

"It is that stability of climate that has allowed our populations to thrive and grow and build on ideas and innovate in a way that is far ahead of anything that our ancestors were able to achieve with smaller numbers," he said.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website. 

NOAA

Concentrations of the greenhouse gas carbon dioxide in air sampled at Mauna Loa Observatory in Hawaii will likely peak above 400 ppm this month, scientists said.

Any day now, the concentration of the greenhouse gas carbon dioxide sampled from the air wafting above a barren lava field in Hawaii could be above 400 parts per million (ppm), a level not seen since the Pliocene, between 3.2 and 5 million years ago.

Carbon dioxide levels were around 280 ppm when the Industrial Revolution got under way in the 18th century and humans started pumping greenhouse gases into the atmosphere from burning fossil fuels. Levels have continued to accelerate higher since then.

Hitting the 400 ppm milestone is symbolic to science and policy discussions about efforts to control global climate change, according Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif. 

Human-forced global climate change is primarily driven by rising concentrations of carbon dioxide. Round numbers such as 350 ppm, 400 ppm and 450 ppm are discussed as targets for "safe" levels of the greenhouse gas - a level that will prevent the climate from spinning out of control, he explained.

"So crossing 400 represents, if you will, a lost opportunity," Keeling said.

His late father, Charles David Keeling, a climate science pioneer at Scripps, began tracking levels of carbon dioxide in 1958 at the Mauna Loa Observatory. Concentrations then were 316 ppm. 

Since then, levels have continued to step higher in a saw-tooth pattern, peaking each May before drifting lower as plants in the Northern Hemisphere absorb carbon dioxide to grow, then rising in the fall and winter.

Keeling Curve
The readings plotted on a graph show carbon dioxide concentrations curving sharply higher over time as humans burn more and more fossil fuels. The graph is known as the Keeling Curve. 

Details from the curve, in fact, show the rate of increase continuing to accelerate, noted Pieter Tans, the senior scientist at the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostic Lab, which oversees the Mauna Loa Observatory.

Scripps Institution of Oceanography

The saw-tooth graph, known as the Keeling Curve, shows rising concentrations of the greenhouse gas carbon dioxide in the atmosphere since record keeping began in 1958.

When David Keeling first started keeping records, the five-year average rate of change was 0.7 ppm. Today, it is 2 ppm, more than three times faster, he said. 

"That is significant," Tans told NBC News. "We are just mostly talking about decreasing emissions. In practice, at least globally average, emissions are still accelerating and so is the rate of CO2 increase in the atmosphere."

But crossing 400 ppm in and of itself "doesn't mean anything in particular," Daniel Sarewitz, a sustainability scientist at Arizona State University, noted in an email to NBC News. "It's just another indicator of what we know full well already — that modern society is enormously dependent on fossil fuels for its well-being."

And that dependence on fossil fuels means concentrations of carbon dioxide will continue to climb for the foreseeable future, he added.

Watching for 400
To mark the crossing of 400 ppm, Scripps Institution of Oceanography launched a website and Twitter feed to provide daily readings from Mauna Loa. As of this writing, it stands at 399.29.

The hourly number fluctuates up and down depending on the atmospheric winds and, to a lesser extent, pockets air that upwell from lower on the island. Scientists calculate a daily average from hourly readings (when there's too much noise, a reading isn't produced). The daily numbers are then averaged to establish weekly, monthly and yearly numbers to be plotted on the graph.

From a scientific point of view, the monthly number matters the most, said Ralph Keeling, who took over the record keeping from his father. It is unlikely that this May will actually have an average above 400 ppm. The month is already a few days old and the threshold hasn't been crossed yet. The peak is typically the middle of the month.

There is, however, a better-than 50-50 chance that at least one daily reading above 400 will be recorded in the next few weeks, he said.

"Of course, we might go the whole month without it, but I'd be very surprised if we don't creep up over 400 next May and in a matter of a couple years, even the trough in the cycle will climb over 400, so it will be very hard to find any air that is below 400," he added.

How high concentrations will eventually go, he added, largely depends how willing humans are to forgo burning available reserves of fossil fuels and how many of those fuels actually exist.

"We are starting to move toward alternate fossil energy — non-conventional fossil fuels like tar sands and so forth," Keeling said. "And that’s a little troubling because it reflects our willingness to expand and use reserves of fossil fuels that probably ought to stay in the ground if we take this problem seriously."

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.

NASA / NOAA

This file NOAA's GOES-13 weather satellite image shows the storm system associated with Superstorm Sandy covering the northeastern United States before landfall.

Six months after Superstorm Sandy killed dozens of people and caused an estimated $50 billion in damage on the East Coast, a majority — 58 percent — of Americans see a connection between recent changes in the weather and global climate change, according to a new report.

"People are beginning to recognize a pattern of extreme weather across the country and are themselves saying 'Aha, I wonder if climate change has something to do with that,'" Anthony Leiserowitz, director of the Yale Project on Climate Change Communication, which released the report today, told NBC News.

About half the country, he added, believes climate change is affecting specific extreme weather events. For example 50 percent linked climate change to the record warmth in 2012, 49 percent to the ongoing drought in the Midwest and Great Plains, 46 percent to Superstorm Sandy, and 42 percent to Superstorm Nemo.

Climate scientists are typically careful not to draw too close of an association between climate change and the day-to-day weather, notes Leiserowitz. The standard talking point is that no single weather event is caused by climate change. Still, climate change may be making the usual weather worse.

"The report provides good evidence for why it is we rely on science rather than public opinion on such matters," Roger Pielke Jr., from the Center for Science and Technology Policy Research at the University of Colorado, said in an email to NBC News. Pielke was not involved with the Yale survey, but has written extensively on climate change policy.

"The attribution of changes in climate on extremes is a difficult and thorny scientific puzzle requiring long-term data," he added. "Unfortunately, the human experience — in one place at a time and over a generation — is not a solid basis for such attribution."

In recent years, however, climate science has evolved to the point where some researchers are beginning to see the fingerprint of climate change on individual heat waves, droughts, hurricanes, and other storms.

Read: Blame blistering heat waves on global warming, study says

These so-called attribution studies calculate the likelihood that events such as the Russian heat wave of 2010 and the drought in Texas and Oklahoma in 2011 could have occurred in the absence of climate change. 

The answer is "very, very small ... so that basic line has begun to shift even among the climate science community," Leiserowitz said.

The American people, he added, "are not empty vessels waiting to be told to think about these issues by scientists or journalists, they are actively interpreting their own experience and what we are seeing in our data is that many Americans are now connecting the dots."

The connections the public is drawing between extreme weather events and climate change are consistent with the climate science, Leiserowitz said. While climate change doesn't necessarily cause the weather events, adding greenhouse gases to the Earth's weather system has an effect similar to giving steroids to a baseball player: Harder hits. It juices the system, he said.

And the picture the connected dots present shows climate change making life harder — about two out of three Americans said the weather has been "worse" over the past couple years, which is up 12 percentage points from 2012. Only 11 percent said the weather has gotten better, down 16 points.

In addition, just over half believe extreme weather will cause a natural disaster in their community in the next year.

That said, only one in three Americans are actually prepared for an extreme weather event, highlighting the need for people to develop an emergency plan and kit and get "ready for the unknown event that is going to happen in your lifetime at some point," Leiserowitz said.

The survey is based on interviews with 1,045 adults between April 8 and 15 with a margin of error of +/- 3 percentage points.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website. 

Darrell S. Kaufman / Northern Arizona University

Kristi Wallace of the Alaska Volcano Observatory examines a lake sediment core from southern Alaska that shows intricate layering indicating environmental and climatic changes over centuries.

A long-term global cooling trend ended in the late 19th century, a reversal in temperature that cannot be explained by natural variability alone, according to a new study.

The finding stems from 2,000-year-long continental-scale temperature records inferred from tree rings, ice cores, lake sediments and other so-called proxies from around the world. 

The records show variations in temperature caused by changes in Earth's orbit, output of solar energy, and volcanic eruptions, noted Nicholas McKay, a climate scientist at Northern Arizona University and study co-author. Volcanic eruptions, for example, inject particles in the atmosphere that reflect some of the solar radiation back out to space.

Read: Warming fastest since dawn of civilization, study shows

"The 18th and 19th centuries would probably have been colder than the 20th century no matter what just because there has been a bit less volcanism in this century, but the amount of warming we've seen is extremely unlikely to have happened solely due to natural processes," he told NBC News.

In fact, he and colleagues note in the study — published Sunday in Nature Geoscience — that the natural factors that drove the Earth's long-term cooling are still present today, despite the fact that we are in a period of rising global temperatures.

The "hockey stick"
The record is consistent with other recent temperature reconstructions that show the reversal in long-term cooling coinciding with the acceleration of greenhouse gas emissions from human activity during the industrial revolution at the end of the 19th century.

Gerald North, a climate scientist at Texas A&M University, told NBC News in an email that the new study seems to fit the emerging consensus of a gradual cooling of the past 1,000 to 2,000 years followed by "an abrupt warming since 1900."

"Each year we have more evidence corroborating these same findings," he said. "It is 15 years since the first paper ... known as the 'hockey stick' paper. We have no credible evidence that they got it wrong."

The researcher behind the iconic 1999 "hockey stick" graph, Pennsylvania State University climate scientist Michael Mann, was not part of the new study, but he told NBC News in an emailed statement that the work of McKay and his team "adds to the growing body of scientific evidence that the recent warming is likely unprecedented even further back in time."

Mann added, "While the study doesn't attribute causality to the warming, there is an extensive body of research that shows that we can only explain the anomalous recent warming with human impacts, i.e. burning of fossil fuels and resulting increase in greenhouse gas concentrations."

Regional temperature variations
One distinguishing feature of the new study, noted McKay, is that it highlights variability in temperature around the globe at any one time. For example, a rise in temperatures known as the Medieval Warm Period followed by cooling during the Little Ice Age was pronounced in Europe and North America, less so in the Southern Hemisphere, he said.

While the paper isn't the first to look at regional climate reconstructions, it is the first so well organized, noted David Anderson, a paleoclimatologist with the National Oceanic and Atmospheric Administration's National Climatic Data Center. And, collectively, the regions show the end to the cooling trend on a global scale. "It is truly no debate," he told NBC News.

The ability to see the regional variability in response to forces on the global climate — from human burning of fossil fuels to volcanic eruptions — will be increasingly important as humans try to mitigate and adapt to future climate change, McKay added.

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.

John Brecher / NBC News

An array of 575 solar panels covers the roof of the Bullitt Center, a role model for a new generation of sustainable, energy-efficient buildings.

In cloudy, drizzly Seattle, Denis Hayes, the environmental activist who organized the first Earth Day in 1970, is pulling the wraps off a six-story office building that generates all of its electricity via an oversized rooftop array of solar panels.

A sun-powered building in Seattle is "formidable," Hayes told NBC News, but the Bullitt Center project aims to show it is possible in a visible, tangible manner that, in turn, makes an impact on the often invisible, slow-motion challenge of global climate change.

"When this whole [Earth Day] thing got launched in 1970, we had people walking around with gas masks and smokestacks were pouring out enormous impenetrable clouds of black smoke," said Hayes, who is now president of the Bullitt Foundation, which supports environmental causes.

Today, the sooty smokestacks and black clouds are largely gone, but our energy demands have never been greater, and the impacts of climate change are considered by environmental advocates to be more and more apparent, ranging from extreme weather to growing food insecurity. As scientists toil to identify how our world is changing, the environmental movement will have to spend the next decades finding better ways to rally the world's citizens to address the new challenges.

Self-sustaining architecture
Hayes says the Bullitt Center addresses many of the environmental issues underpinned by global climate change. Take, for example, the building's 56,000-gallon basement cistern for storing captured rainwater. It's a seemingly odd fixture for a building in a city famous for its rain and amply supplied by water, stored in snow form, from the nearby Cascade Mountains.

Slideshow: A look at the world's greenest office building

John Stamets

The Bullitt Foundation, whose mission is to safeguard the environment, spent $18.5 million to construct this uniquely sustainable office building.

Launch slideshow

"There is not going to be enough water in the future," Hayes said. "There is going to be far less snowpack. The water is going to come in gushers and there is just no way to build additional reservoirs to capture it in the Cascades."

John Brecher / NBC News

Hayes, who co-founded Earth Day with Gaylord Nelson, now heads the Seattle-based Bullitt Foundation.

The solution? Build thousands of reservoirs in the basements of buildings, sufficient to hold water to meet the needs of the current and future residents of a city that studies suggest could grow by at least a million people in the next few decades — potentially, many climate refugees from the parched Southwest, noted Hayes.

The hope is that "by trying to build a building as best we could, that literally does everything right," the Bullitt Center will drive change in the building industry with the type of impact Tom's of Maine, a green personal care products company, had on Proctor and Gamble, an industry giant, Hayes said.

Given that buildings in the U.S. account for 39 percent of the nation's carbon dioxide emissions, 65 percent of waste and 70 percent electrical use, adoption of the Living Building Challenge standards highlighted by the Bullitt Center could make life on Earth more sustainable by the middle of this century.

"If we can get a whole bunch of these other buildings built and we start eating up that market share, then we can begin to have changes that are not small incremental one inch here, five inches there, but a fairly profound impact," he explained. "At least, that's the theory."

Want to know more about the Bullitt Center? Read our feature story here, and check out our slideshow.

Rising food insecurity
But even if self-sustainable architecture really takes off, it can't solve all the environmental issues, Hayes noted. In particular, he worries about food security, at a time when rising prosperity around the world means a shift to more meat-heavy diets. 

The shift in diet is not the only issue, either. It may be hard enough to meet the basic needs of a hungry population, according to Lester Brown, founder and president of the Earth Policy Institute and a leading thinker on global environmental issues.

"Agriculture as it exists today has evolved over an 11,000-year period of rather remarkable climate stability," he told NBC News. "If the climate system begins to change, suddenly the climate system and the agricultural system will no longer be in sync with each other. With each passing year, they will be more and more out of sync, making it difficult to just maintain production, much less to increase it."

The solution, Brown added, would be to escalate the problem from the agriculture departments and ministries up to the very heads of state, who will need to grasp food's relationship to everything from energy policy to shifts in population.

And in turn, world leaders need to work together. 

"No one country can ensure food security on its own because we are talking about climate change being one of the keys here and that requires a global effort," Brown explained. "No country can stabilize its climate unilaterally; it takes a concerted international effort."

Rousing international cooperation
Building an international coalition to tackle global climate change is plausible, and happening, leading environmental scientists said Thursday during a panel discussion with recipients of the Tyler Prize, an environmental achievement award, at the National Press Club in Washington, D.C.

For example, John Holdren, director of the White House Office of Science and Technology and a Tyler Prize laureate in 2000, said that in both India and China "the leadership is fully aware of the damage already being done to their countries by climate change and they are fully aware that ultimately the problem cannot be solved without their participation."

That participation, he added, is already underway, as exhibited by a Joint U.S.-China Statement on Climate Change posted on the Department of State's website earlier this month.

"Forceful, nationally appropriate action by the United States and China — including large-scale cooperative action — is more critical than ever," reads the statement. "Such action is crucial both to contain climate change and to set the kind of powerful example that can inspire the world."

Only time will tell if the statement is truly the beginning of something focused and concrete that puts the world on the path to tackling the biggest environmental issues of today and tomorrow.

Meanwhile, Hayes, on the 43rd anniversary of the first Earth Day rally, readies his new self-sustaining office building for the world to see, as he continues to search for ways to raise awareness about the environment.

"Somehow we have to take all of these issues — and I don't have the magic formula — and make them the most immediate concern," he said. "And I guess the reason for optimism is that sometimes stuff just happens. It is almost like a school of fish — it is a biological phenomenon."

John Roach is a contributing writer for NBC News. To learn more about him, visit his website.

Andrew Kemp, Yale University

Sea level rise is swamping coasts; Rodanthe in the Outer Banks of North Carolina is pictured.

By Becky Oskin
LiveScience

Sharp reductions in short-lived airborne pollutants could significantly slow sea level rise before 2100, a new study finds.

The four pollutants — black carbon, methane, ozone and hydrofluorocarbons — all cycle through the atmosphere more quickly than carbon dioxide, which lasts for centuries in the troposphere, the part of the atmosphere we live in and breathe. Carbon dioxide is the main culprit in Earth's warming temperatures, which impacts sea level rise both by the expansion of water as it warms and by the melting of glacial ice.

Cutting the air pollutants, which all also act to trap heat in the atmosphere and last anywhere from a week to decade, worldwide by 30 percent to 60 percent over the next several decades would lower predicted sea level rise by 22 percent to 42 percent by 2100, according to the study, published Sunday in the journal Nature Climate Change.

Sea levels are expected to rise between 7 inches to 6.6 feet (18 centimeters to 2 meters) this century, according to a 2007 assessment by the Intergovernmental Panel on Climate Change. The higher tides will bring more coastal flooding and bigger storm surges, the IPCC report warned.

Though the four pollutants are known contributors to climate change, policymakers tend to focus on carbon dioxide, the 800-pound-gorilla of global warming, when it comes to reducing emissions. Frustrated at the slow pace of negotiations on cutting carbon dioxide, the research team decided to investigate other ways to slow the planet's warming, according to a statement from the National Center for Atmospheric Research (NCAR) in Boulder, which participated in the research.

"To avoid potentially dangerous sea level rise, we could cut emissions of short-lived pollutants even if we cannot immediately cut carbon dioxide emissions," NCAR's Aixue Hu, lead study author, said in the statement. "This new research shows that society can significantly reduce the threat to coastal cities if it moves quickly on a handful of pollutants."

The study models relied on emissions cuts beginning in 2015. Hu and his colleagues tested the effects of lowering atmospheric levels of the four gases and particles by 30 percent to 60 percent over the next several decades, the steepest cuts economists believed possible, the study said.

Even if these cuts are made, though, carbon dioxide is still the main threat, the authors said.

"It must be remembered that carbon dioxide is still the most important factor in sea level rise over the long term," Warren Washington, a study co-author at NCAR, said in the statement. "But we can make a real difference in the next several decades by reducing other emissions."

Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook and Google+. Original article on LiveScience.com.

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Jeremy Potter NOAA / OAR / OER

The dramatic loss of Arctic sea ice in summer is just one of the signs global warming has not stopped, scientists say.

By Becky Oskin
LiveScience

By the time today's babies graduate college, there's a very good chance they could celebrate with a cruise across the North Pole.

That's according to the latest study on Arctic summer sea ice, the frozen pack that lingers through the Northern Hemisphere summer. In past decades, there's been less summer ice, and it's growing thinner.

The research, published online Feb. 21 in the journal Geophysical Research Letters, says major sea ice loss could come within a decade or two, though some ice will stick around near Greenland and Canada's Arctic islands.

NSIDC

Arctic sea ice at the end of melt season, 1981-2009.

The results come from analyzing different approaches to predicting Arctic sea-ice melt. Researchers James Overland, of the National Oceanic and Atmospheric Administration, and Muyin Wang, of the University of Washington, looked at three common modeling techniques to come up with the best forecast for when the Arctic will be ice-free in the summer.

"There is no one perfect way to predict summer sea ice loss in the Arctic," Wang said in a statement. "So we looked at three approaches that result in widely different dates, but all three suggest nearly sea ice-free summers in the Arctic before the middle of this century," she said.

Here are brief descriptions of the three modeling styles the study takes on:

• Models relying on past sea ice trends, or changes in total amount of sea ice, predict a nearly ice-free Arctic by 2020.
• A more stochastic approach, which incorporates random large sea-ice melting events, like the big ice losses in 2007 and 2012. This model suggests the Arctic could be ice-free by 2030.
• The climate modelers, who rely on global climate information to gauge Arctic warming, estimate sea-ice loss will hit around 2060. This is likely too slow, Overland and Wang report in their study.

No matter the model, it's reasonable to conclude that a nearly ice-free Arctic summer will very likely occur before 2050, and possibly by 2025 or 2035 (in a decade or two), Overland and Wang said in the study.

Less Arctic summer ice will have global impacts, from opening shipping lanes and exploration for resources such as oil and gas to further warming the Arctic by exposing more ocean to the sun's warmth (open ocean absorbs the sun's rays, while ice reflects them). Arctic warming is also changing the jet stream's pattern, with steeper waves that bring extreme weather to lower latitudes, studies show.

"Rapid Arctic sea ice loss is probably the most visible indicator of global climate change; it leads to shifts in ecosystems and economic access, and potentially impacts weather throughout the Northern Hemisphere," Overland said in a statement.

Email Becky Oskin or follow her @beckyoskin. Follow us @OAPlanet, Facebook or Google+. Original article on LiveScience's OurAmazingPlanet.

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