Planning a trip to Mars? Take plenty of shielding. According to sensors on NASA's ACE (Advanced Composition Explorer) spacecraft, galactic cosmic rays have just hit a Space Age high.

"In 2009, cosmic ray intensities have increased 19% beyond anything we've seen in the past 50 years," says Richard Mewaldt of Caltech. "The increase is significant, and it could mean we need to re-think how much radiation shielding astronauts take with them on deep-space missions."

The cause of the surge is solar minimum, a deep lull in solar activity that began around 2007 and continues today. Researchers have long known that cosmic rays go up when solar activity goes down. Right now solar activity is as weak as it has been in modern times, setting the stage for what Mewaldt calls "a perfect storm of cosmic rays."

"We're experiencing the deepest solar minimum in nearly a century," says Dean Pesnell of the Goddard Space Flight Center, "so it is no surprise that cosmic rays are at record levels for the Space Age."

Galactic cosmic rays come from outside the solar system. They are subatomic particles--mainly protons but also some heavy nuclei--accelerated to almost light speed by distant supernova explosions. Cosmic rays cause "air showers" of secondary particles when they hit Earth's atmosphere; they pose a health hazard to astronauts; and a single cosmic ray can disable a satellite if it hits an unlucky integrated circuit.

The sun's magnetic field is our first line of defense against these highly-charged, energetic particles. The entire solar system from Mercury to Pluto and beyond is surrounded by a bubble of solar magnetism called "the heliosphere." It springs from the sun's inner magnetic dynamo and is inflated to gargantuan proportions by the solar wind. When a cosmic ray tries to enter the solar system, it must fight through the heliosphere's outer layers; and if it makes it inside, there is a thicket of magnetic fields waiting to scatter and deflect the intruder.

"At times of low solar activity, this natural shielding is weakened, and more cosmic rays are able to reach the inner solar system," explains Pesnell.

Mewaldt lists three aspects of the current solar minimum that are combining to create the perfect storm:

1. The sun's magnetic field is weak. "There has been a sharp decline in the sun's interplanetary magnetic field (IMF) down to only 4 nanoTesla (nT) from typical values of 6 to 8 nT," he says. "This record-low IMF undoubtedly contributes to the record-high cosmic ray fluxes."
2. The solar wind is flagging. "Measurements by the Ulysses spacecraft show that solar wind pressure is at a 50-year low," he continues, "so the magnetic bubble that protects the solar system is not being inflated as much as usual." A smaller bubble gives cosmic rays a shorter-shot into the solar system. Once a cosmic ray enters the solar system, it must "swim upstream" against the solar wind. Solar wind speeds have dropped to very low levels in 2008 and 2009, making it easier than usual for a cosmic ray to proceed.
3. The current sheet is flattening. Imagine the sun wearing a ballerina's skirt as wide as the entire solar system with an electrical current flowing along the wavy folds. That is the "heliospheric current sheet," a vast transition zone where the polarity of the sun's magnetic field changes from plus (north) to minus (south). The current sheet is important because cosmic rays tend to be guided by its folds. Lately, the current sheet has been flattening itself out, allowing cosmic rays more direct access to the inner solar system.

Since May 2009, the tropical Pacific Ocean has switched from a cool pattern of ocean circulation known as La Niña to her warmer sibling, El Niño. This cyclical warming of the ocean waters in the central and eastern tropical Pacific generally occurs every three to seven years, and is linked with changes in the strength of the trade winds. El Niño can affect weather worldwide, including the Atlantic hurricane season, Asian monsoon season and northern hemisphere winter storm season. But while scientists agree that El Niño is back, there's less consensus about its future strength.

One of the characteristics that signal a developing El Niño is a change in average sea surface height compared to normal sea level. The NASA/French Space Agency Jason-1 and Ocean Surface Topography Mission/Jason-2 satellites continuously observe these changes in average sea surface height, producing near-global maps of the ocean's surface topography every 10 days.

Recent data on sea-level height from the Jason-1 and Ocean Surface Topography Mission/Jason-2 satellites, displayed at http://sealevel.jpl.nasa.gov/science/jason1-quick-look/ , show that most of the equatorial Pacific is near normal (depicted in the images as green). The exceptions are the central and eastern equatorial Pacific, which are exhibiting areas of higher-than-normal sea surface heights (warmer-than-normal sea-surface temperatures) at 180 and 110 degrees west longitude.

The latest image from Jason-2, which can be seen at http://sealevel.jpl.nasa.gov/science/jason1-quick-look/2009/images/20090917P.jpg, reflects a 10-day data cycle centered around September 17, 2009. It shows a series of warm "bumps" visible along the equator, denoted in the image by a black line. Known as Kelvin waves, these pools of warm water were triggered when the normally westward-blowing trade winds weakened in late July and again in early September, sending them sliding eastward from the western Pacific toward the Americas. The Kelvin waves are 5 to 10 centimeters (2 to 4 inches) high, a few hundred kilometers wide and a few degrees warmer than surrounding waters. Traveling east at about 3 meters per second (6 miles per hour), they are expected to reach the coast of Peru in October. (An animation of the evolution of Pacific Ocean conditions since January 2006 is at: http://www.jpl.nasa.gov/videos/earth/elnino20090928.mov ).

Yet the present condition of this year's El Niño is dwarfed in comparison with the "macho" El Niño of 1997-1998, which brought devastating floods to California and severe drought to Indonesia, Australia and the Philippines. As seen in this September 20, 1997, image from the NASA/French Space Agency Topex/Poseidon satellite (see http://sealevel.jpl.nasa.gov/files/images/browse/entp2090.gif ), the size and intensity of the 1997-1998 event were much greater by this time of year. That leads some scientists, such as Bill Patzert, an oceanographer and climatologist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., to express uncertainty as to whether this El Niño event will intensify enough to deliver the dramatic impacts seen during that last intense El Niño in 1997-1998.

"For the past few months, the trade winds have weakened somewhat, but whether the new Kelvin waves traveling eastward across the Pacific will be adequate to pump this El Niño up enough to reinvigorate it and deliver any real impacts remains uncertain," Patzert says.

Patzert notes that it is important to remember that not all El Niños are created equal. "Some El Niños are show stoppers, but most are mild to modest, with minimal to mixed impacts," he says. He notes that since 1998, there have been three mild to moderate El Niño's: in 2002-2003, 2004-2005 and 2006-2007.

None of these events delivered the heart-thumping impacts of the monster El Niño of 1997-1998. During the winter of 1997-1998, Southern California was soaked with nearly 79 centimeters (more than 31 inches) of rain (twice Los Angeles' normal annual rainfall amount of about 38.5 centimeters, or 15.14 inches). In addition, there was heavy snowpack in the Sierra Nevada and Rocky Mountains. In comparison, during the past four winters, Los Angeles has averaged only 24.6 centimeters (9.7 inches) of rain (64 percent of normal), and snowpacks have been stingy.

In fact, Patzert notes that this El Niño bears many similarities to the 2006-2007 El Niño event. During that winter, much of the American Southwest experienced record-breaking drought, and Los Angeles had its driest winter in recorded history.

So what will El Niño 2009-2010 hold in store for the world this coming winter? In spite of the uncertainties, experienced climate forecasters around the world will continue to monitor the Pacific closely for further signs of El Niño development and will give it their best shot.

"Unless present El Niño conditions intensify, I believe this El Niño is too weak to have a major influence on many weather patterns," he says. "A macho El Niño like that of 1997-1998 is off the board, but I'm hoping for a relaxation in the tropical trade winds and a surprise strengthening of El Niño that could result in a shift in winter storm patterns over the United States. If the trade winds decrease, the ocean waters will continue to warm and spread eastward, strengthening the El Niño. That scenario could bring atmospheric patterns that will deliver much-needed rainfall to the southwestern United States this winter. If not, the dice seem to be loaded for below-normal snowpacks and another drier-than-normal winter."

Still, Patzert remains hopeful. "Don't give up on this El Niño," he added. "He might make a late break and put his spin on this fall and winter's weather systems."

To learn more about Jason-1 and the Ocean Surface Topography Mission/Jason-2, visit: http://sealevel.jpl.nasa.gov .

In their recently published research paper² entitled "Is the climate warming or cooling?", David Easterling of the U.S. National Climate Data Center and Michael Wehner of Lawrence Berkeley National Laboratory show that naturally occurring periods of no warming or even slight cooling can easily be part of a longer-term pattern of global warming.

This may sound counter-intuitive at first sight, so let's take a closer look at the data. Figure 1 shows the change in the world's air temperature averaged over all the land and ocean between 1975 and 2008. The warming is obvious -- about 0.5° C (0.9° F) during that time. However, there are plenty of periods -- 1997 to 1985 and 1981 to 1989 (see insets, Figure 1), and 1998 to 2008 -- when no warming is seen, the most recent of which some global warming skeptics say is evidence that the world is actually cooling.

What's going on? To answer this question, Easterling and Wehner pored over global temperature records dating from 1901 to 2008 and also ran computer simulations of Earth's climate looking back into the past and forward into the future. They concluded that in a climate being warmed by man-made carbon emissions, "it is possible, and indeed likely, to have a period as long as a decade or two of 'cooling' or no warming superimposed on a longer-term warming trend."

Natural Fluctuations

These temperature plateaus, or cooling spells, can be attributed to natural climate variability, explains Josh Willis, a climate scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. and a recent recipient of the 2009 Presidential Early Career Award for Scientists and Engineers. "Natural variability refers to naturally-occurring fluctuations or events that change Earth's climate on time scales ranging from years to decades. Big volcanic eruptions, for instance, can cause cooling that lasts for several years. When a volcano erupts, it blasts dust into the upper atmosphere where it reflects sunlight and cools the planet, a bit like a natural umbrella." He goes on, "There are also all kinds of natural fluctuations that sometimes cause warming, sometimes cooling." Ocean changes, for instance, can have a big impact on the world's temperature. One example that Willis cites is the Pacific Decadal Oscillation, a pattern of warmer and cooler surface temperatures in the Pacific Ocean that can last between 10 and 30 years.

The authors say that it is easy to "cherry-pick" a period to reinforce a particular point of view. "Claims that global warming is not occurring that are derived from a cooling observed over short time periods ignore natural variability and are misleading."

What you have to look at, says Willis, is the long-term temperature readings that have been collected over the past century -- which is exactly what Easterling and Wehner do in their study. Over that sort of time scale, global warming becomes apparent from observations of both our atmosphere and our ocean, which are intimately linked pieces of the climate puzzle.

Sea Change

Since around the time of the Industrial Revolution (the late 18th and early 19th centuries), Earth's atmosphere has warmed by a little less than 1° C (1.8° F) (Figure 2). In turn, the ocean has also risen by about 15 centimeters (6 inches) over the past 100 years -- for two reasons. First, when water warms up, it expands, in much the same way as a solid does when it heats up. As the volume of seawater increases, it causes sea level to rise. Second, global warming causes glaciers and ice sheets to melt, which adds more water to the world's ocean, again causing sea level to rise ^4,5.

"If you look at the ocean data, there has been a very clear acceleration in sea level rise," explains Willis. "At the beginning of the last century, sea level was rising by less than 1 millimeter (0.04 inches) per year; mid-century it was 2 millimeters (0.08 inches) per year and now it's 3 millimeters (0.12 inches) per year. This is directly caused by the increasing temperature of the planet."

Big Picture

As Willis explains, global warming is a long-term process. "Despite the fact it's been warmer and cooler at different times in the last 10 years, there's no part of the last 10 years that isn't warmer than the temperatures we saw 100 years ago."

Assuming our greenhouse gas emissions continue at their present levels with little reduction, existing climate forecasts suggest that our planet will warm by about 4° C (7.2° F) by the end of the 21st century. Although scientists continue to study the nuances of Earth's climate, the link between carbon emissions, global warming and sea level rise over the past century is clear. Even if our global carbon emissions began to fall tomorrow, Earth would continue to warm for some time due to the inertia of the climate system.

"In the next century it's definitely going to get warmer," Willis says. "You don't need a crystal ball or fancy climate model to say that. Just look at the sea level and temperature records from the past 100 years -- they're all going up." Likewise, Easterling and Wehner's work reminds us that understanding climate change -- one of the most important challenges we face today -- requires a long-term view. "Unlike people," says Willis, "the climate has a very long memory."

A Body of Evidence

In 2007, a scientific intergovernmental body called the Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report on Climate Change, which summarizes our current understanding of climate change. The report took 6 years to produce, involved over 2500 scientific expert reviewers and more than 800 authors from over 130 countries.

Some of their key findings include:

• The warming trend over the last 50 years (about 0.13° C or 0.23° F per decade) is nearly twice that for the last 100 years.
• The average amount of water vapor in the atmosphere has increased since at least the 1980s over land and ocean. The increase is broadly consistent with the extra water vapor that warmer air can hold.
• Since 1961, the average temperature of the global ocean down to depths of at least 3 km (1.9 miles) has increased. The ocean has been absorbing more than 80% of the heat added to the climate system, causing seawater to expand and contributing to sea level rise.
• Global average sea level rose on average by 1.8 mm (0.07 inches) per year from 1961 to 2003. There is high confidence that the rate of observed sea level rise increased from the 19th to the 20th century.
• Average arctic temperatures increased at almost twice the global average rate in the past 100 years.
• Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise.
• Long-term trends in the amount of precipitation have been observed over many large regions from 1900 to 2005.