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The concept of time as a way to measure the duration of events is not only deeply intuitive, it also plays an important role in our mathematical descriptions of physical systems. For instance, we define an object's speed as its displacement per a given time. But some researchers theorize that this Newtonian idea of time as an absolute quantity that flows on its own, along with the idea that time is the fourth dimension of spacetime, are incorrect. They propose to replace these concepts of time with a view that corresponds more accurately to the physical world: time as a measure of the numerical order of change.

By taking advantage of the properties of periodic systems, physicists have described how to efficiently time-reverse ultrashort electromagnetic pulses. Since a time-reversed pulse evolves as if time runs backwards, time reversal eliminates any distortions or scattering that occurred at earlier times, regardless of the medium the pulse has propagated through.

Back in 2009 Mayor Bloomberg prohibited vehicle traffic on Broadway between 42nd Street and 47th Street in Times Square to create a pedestrian plaza (video here). Now, the city's most recent Community Air Survey found that, "After the conversion to a pedestrian plaza, NO pollution levels in Times Square went down by 63 percent while, NO2 levels went down by 41 percent." Unlike N2O, which makes you laugh at Phish shows, NO2, or nitrogen dioxide, destroys your lungs and may cause Sudden Infant Death Syndrome. Nitric oxide (NO) isn't good for you either.

David Keith is Gordon McKay Professor of Applied Physics at Harvard's School of Engineering and Applied Sciences, and Professor of Public Policy at Harvard Kennedy School. The award-winning scientist, who was named one of TIME magazine's Heroes of the Environment in 2009, has worked near the interface of climate science, energy technology, and public policy for twenty years. He divides his time between Boston and Calgary, where he serves as president of Carbon Engineering-a start-up company developing industrial-scale technologies for capture of CO2 from ambient air. Here, Keith answers questions about his research and ideas for reducing climate change using innovative and sometimes controversial methods.

The rate of release of carbon into the atmosphere today is nearly 10 times as fast as during the Paleocene-Eocene Thermal Maximum (PETM), 55.9 million years ago, the best analog we have for current global warming, according to an international team of geologists. Rate matters and this current rapid change may not allow sufficient time for the biological environment to adjust.

Mammoths were one of the "megafauna" that went extinct during the Pleistocene extinction. According to the first study of mammal range and diversity in "deep time" from the Eocene to the Pleistocene, the mammoth family did not show any sign of being predisposed for extinction.

For the first time, the melting of glaciers in Greenland could now be measured with high accuracy from space. Just in time for the tenth anniversary of the twin satellites GRACE (Gravity Recovery and Climate Experiment) a sharp image has surface, which also renders the spatial distribution of the glacial melt more precisely. The Greenland ice shield had to cope with up to 240 gigatons of mass loss between 2002 and 2011. This corresponds to a sea level rise of about 0.7 mm per year. These statements were made possible by the high-precision measurements of the GRACE mission, whose data records result in a hitherto unequaled accurate picture of the earth's gravity. One of Newton's laws states that the gravity of an object depends directly on its mass. "When the mass of the Greenland ice sheet changes, so does the gravity there," explains Dr. Frank Flechtner from the GFZ German Research Centre for Geosciences. "The GRACE gravity field measurements therefore give us information on mass changes, including climate-related ones."

Steven Pinker charts the decline of violence from Biblical times to the present, and argues that, though it may seem illogical and even obscene, given Iraq and Darfur, we are living in the most peaceful time in our species' existence.

A recently released report of the General Social Survey, conducted by NORC at the University of Chicago, shows that for the first time since 1972, more Americans say that their financial situation has gotten worse in recent years rather than better. Understandably, also for the first time since 1972, the percentage of Americans saying that they are "not at all" satisfied with their financial situation (31.5%) notably exceeds those saying they are "pretty well" satisfied (23.4%).

There is a vitally important national and state-level discussion taking place about how to address the government's frighteningly large deficits while limiting disruptions for all Americans. But while federal and state actions affect us all, much of what concerns Americans most directly and personally happens closer to home and increasingly, this means in our cities. Budget cuts simply can't derail efforts to make our cities smarter. Today our nation's cities are facing a "perfect storm." For many cities, their populations are growing at the same time that their budgets are shrinking. Today, for the first time, more than 50% of the world's population lives in cities. In the last decade large metropolitan areas in the U.S. grew by a combined 10% - nearly double the rate of the rest of the country. Our large metro areas now house two thirds of America's total population. They have become the dominant forces in our economy and society.

After six years of intensive effort, scientists are reporting development of the first commercially viable nanogenerator, a flexible chip that can use body movements - a finger pinch now en route to a pulse beat in the future - to generate electricity. Speaking here today at the 241st National Meeting & Exposition of the American Chemical Society, they described boosting the device's power output by thousands times and its voltage by 150 times to finally move it out of the lab and toward everyday life.

"A new study has found that climate-prediction models are good at predicting long-term climate patterns on a global scale but lose their edge when applied to time frames shorter than three decades and on sub-continental scales."

Bird migration timing across North America has been affected by climate change, according to a study published Feb. 22 in the open access journal PLoS ONE.

The time may finally be right to sell Americans on eco-driving, according to a group of transportation experts from four University of California campuses as well as representatives from industry and government who attended an all-day conference on May 18.

A recent study of the Catalina Eddy performed by Kanamitsu. The figure shows the 3-hourly evolution of the eddy during two days. Kanamitsu discovered that the eddy disappears during 00Z and 03Z, which had never been reported before. This was due to the lack of high time-resolution observations. This kind of analysis is only possible using the dynamically downscaled analysis

Smartphone apps may be the key to getting people out of their cars and onto mass transit. An interesting study of commuters in Boston and San Francisco found people are more willing to ride the bus or train when they have tools to manage their commutes effectively. The study asked 18 people to surrender their cars for one week. The participants found that any autonomy lost by handing over their keys could be regained through apps providing real-time information about transit schedules, delays and shops and services along the routes. Though the sample size is small, the researchers dug deep into participants' reactions. The results could have a dramatic effect on public transportation planning, and certainly will catch the attention of planners and programmers alike. By encouraging the development of apps that make commuting easier, transit agencies can drastically, and at little cost, improve the ridership experience and make riding mass transit more attractive.

Researchers in America have shown that ozone -- a known pollutant at low levels in the earth's atmosphere, causing harmful effects on the respiratory system and sensitive plants -- can be reduced, on average, when electric vehicle charging is done at night time.

According to a series of articles published in Nature, the world has too many PhDs and not enough academic jobs to sustain them. Researchers point out that it is either time to make changes in the system or eliminate it altogether.

"Increases in humidity caused as a result of climate change are reducing labour productivity and it's only likely to get worse over time, argue researchers from America's National Oceanic and Atmospheric Administration."

Parasitic worms that infect fish, and have a devastating effect on fish reproduction, grow four times faster at higher temperatures - providing some of the first evidence that global warming affects the interactions between parasites and their hosts.