The past decade has brought the dangers of climate change into sharp relief, often most clearly through images.
In the Arctic, scientific expeditions this year found increasingly thin ice and surprisingly open seas. Higher up, photographers documented the disappearance of glaciers, including some in the Andes and the Himalayas that provide fresh water to billions of people. On lower lands, drought threatened crops and lives from China to Kenya, Australia to California.
Here's a look at some of the most worrisome environmental changes through the lenses of scientists, satellites, explorers and humanitarians.
Arctic Sea Ice
Scientists with the National Snow and Ice Data Center grew increasingly concerned this year as satellite images revealed thinner sea ice covering less of the Arctic. While this opens routes to shipping and natural resources, it poses a danger for the region's wildlife and the climate as we know it.
NSIDC's Tom Wagner compares the role of Arctic sea ice to a giant air conditioner. The ice reflects sunlight back into space and keeps the region cooler by preventing the dark seawater from absorbing that heat, which in turn keeps ocean systems in balance.
This November, the average extent of Arctic sea ice was 3.96 million square miles, about 405,000 square miles less than the 1979 to 2000 average for Novembers, according to NSIDC. In the past, ice thick enough to survive the summers made up 30 to 40 percent of the Arctic's sea ice. Now, it makes up about 10 percent; the rest is younger ice that is more prone to melting.
Photographers documented glacial retreat in Europe, South America, North America and Asia. In the Andes, the World Bank warned that the loss of glaciers like Bolivia's Chacaltaya threaten economic development and basic freshwater supplies. In the Andes region, for example, World Bank researchers noted the glaciers had lost 20 percent of their volume since 1970.
"Melting glaciers will have serious consequences for Andean cities' water supply, with 77 million people expected to be under severe water stress by 2020," World Bank researchers wrote.
They explained that many Andean countries depend on hydropower — it accounts for about 50 percent of the power supply in Ecuador, and close to 70 percent in Bolivia and Peru. An Oxfam International study released earlier this year found that limited freshwater supplies due to shrinking glaciers in the Andes was already deepening water conflicts.
Right: The Peruvian village where Cayetano, a farmer from Ocongate district near Cusco, and his family live has depended on annual melt water from the Ausangate glacier for generations. The shrinking of the Andean glaciers has led to water shortages that are now affecting his crops. Scientists predict that in the next 20 to 30 years, several of those glaciers will disappear, as Chacaltaya did this year.
Drought marked 2009 across several continents. The seasonal rains failed again in parts of east Africa stretching into Kenya, Uganda (right), Ethiopia and Somalia. The lack of water for crops and livestock worsened tensions in the Horn of Africa, the UN reported.
Crops in parts of the Middle East and Central Asia were also parched by one of that region's worst droughts in recent history. In northern China, drought left nearly 5 million people without enough water this summer.
Below: The Aral Sea in Uzbekistan and Kazakhstan, once the world's fourth largest lake, has been depleted by a different source: farmers siphoning off water from the rivers that feed it. Scientists say the loss of the huge body of water has caused a noticeable effect on the climate, with shorter growing seasons and more wind-blow dust from the dried lake bed.
(Photos: Rice in drought, IRRI; Arctic Ice, Nick Cobbing/Greenpeace; Peruvian farmers Oxfam/Flickr; Drought in Ethiopia, Zeresenay Berhane Mehar/Oxfam; Drought in Uganda, James Akena/ Oxfam; More year-to-year images are available through NASA's Earth Observatory, which has been documenting the changing planet for the past 10 years.)