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<jats:p>For the past several decades, oceanographers have embraced the dominant paradigm that the ocean’s meridional overturning circulation operates like a conveyor belt, transporting cold waters equatorward at depth and warm waters poleward at the surface. Within this paradigm, the conveyor, driven by changes in deepwater production at high latitudes, moves deep waters and their attendant properties continuously along western boundary currents and returns surface waters unimpeded to deepwater formation sites. A number of studies conducted over the past few years have challenged this paradigm by revealing the vital role of the ocean’s eddy and wind fields in establishing the structure and variability of the ocean’s overturning. Here, we review those studies and discuss how they have collectively changed our view of the simple conveyor-belt model.</jats:p>

<jats:p>Climate change, rising atmospheric carbon dioxide, excess nutrient inputs, and pollution in its many forms are fundamentally altering the chemistry of the ocean, often on a global scale and, in some cases, at rates greatly exceeding those in the historical and recent geological record. Major observed trends include a shift in the acid-base chemistry of seawater, reduced subsurface oxygen both in near-shore coastal water and in the open ocean, rising coastal nitrogen levels, and widespread increase in mercury and persistent organic pollutants. Most of these perturbations, tied either directly or indirectly to human fossil fuel combustion, fertilizer use, and industrial activity, are projected to grow in coming decades, resulting in increasing negative impacts on ocean biota and marine resources.</jats:p>

<jats:p>Global sea levels have risen through the 20th century. These rises will almost certainly accelerate through the 21st century and beyond because of global warming, but their magnitude remains uncertain. Key uncertainties include the possible role of the Greenland and West Antarctic ice sheets and the amplitude of regional changes in sea level. In many areas, nonclimatic components of relative sea-level change (mainly subsidence) can also be locally appreciable. Although the impacts of sea-level rise are potentially large, the application and success of adaptation are large uncertainties that require more assessment and consideration.</jats:p>

<jats:p>Climate change will alter marine ecosystems; however, the complexity of the food webs, combined with chronic undersampling, constrains efforts to predict their future and to optimally manage and protect marine resources. Sustained observations at the West Antarctic Peninsula show that in this region, rapid environmental change has coincided with shifts in the food web, from its base up to apex predators. New strategies will be required to gain further insight into how the marine climate system has influenced such changes and how it will do so in the future. Robotic networks, satellites, ships, and instruments mounted on animals and ice will collect data needed to improve numerical models that can then be used to study the future of polar ecosystems as climate change progresses.</jats:p>

<jats:p>Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed. Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation. The impacts of anthropogenic climate change so far include decreased ocean productivity, altered food web dynamics, reduced abundance of habitat-forming species, shifting species distributions, and a greater incidence of disease. Although there is considerable uncertainty about the spatial and temporal details, climate change is clearly and fundamentally altering ocean ecosystems. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.</jats:p>

<jats:p>Surveillance of pigs is important for tracking reassortment and emergence of influenza A viruses.</jats:p>

<jats:title>Birth of the Cool</jats:title> <jats:p> Over the past 4 million years or so, tropical sea surface temperatures have experienced a cooling trend (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5985" page="1488" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1189748">Philander</jats:related-article> </jats:bold> ). <jats:bold> Herbert <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1530" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185435">1530</jats:related-article> ) analyzed sea surface temperature records of the past 3.5 million years from low-latitude sites spanning the world's major ocean basins in order to determine the timing and magnitude of the cooling that has accompanied the intensification of Northern Hemisphere ice ages since the Pliocene. <jats:bold> Martínez-Garcia <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1550" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1184480">1550</jats:related-article> ) found that the enigmatic eastern equatorial Pacific cold tongue, a feature one might not expect to find in such a warm region receiving so much sunlight, first appeared between 1.8 and 1.2 million years ago. Its appearance was probably in response to a general shrinking of the tropical warm water pool caused by general climate cooling driven by changes in Earth's orbit. </jats:p>

<jats:title>Close, But Not Too Close</jats:title> <jats:p> MicroRNAs (miRNAs) in plants are generally highly complementary to their target RNAs, yet, in most animal miRNAs, only the ∼8-nucleotide “seeds” sequence bases pair fully with the target, with few base pairs between the remainder of the miRNA and target. Plant miRNAs are methylated at their 3′ ends, whereas animals' miRNAs are not. <jats:bold> Ameres <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1534" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1187058">1534</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5985" page="1494" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1191531">Pasquinelli</jats:related-article> </jats:bold> ) noticed that, in fruit flies, miRNAs engineered to have high complementarity to target RNAs were present at reduced levels. These miRNAs were trimmed and uridylated at their 3′ ends, features involved in RNA degradation. Fly small interfering RNAs, all of which are methylated at their 3′ ends, were unaffected, unless the methylating enzyme, Hen1, was mutated. Thus, 3′-methylation may prevent complementarity-driven remodeling and degradation of small RNAs. </jats:p>

<jats:title>Going Down the Tube</jats:title> <jats:p> Two pillars of modern physics are quantum mechanics and general relativity. So far, both have remained apart with no quantum mechanical description of gravity available. <jats:bold> Van Zoest <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1540" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1189164">1540</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5985" page="1491" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1191666">Nussenzveig and Barata</jats:related-article> </jats:bold> ) present work with a macroscopic quantum mechanical system—a Bose-Einstein condensate (BEC) of rubidium atoms in which the cloud of atoms is cooled into a collective quantum state—in microgravity. By dropping the BEC down a 146-meter-long drop chamber and monitoring the expansion of the quantum gas under these microgravity conditions, the authors provide a proof-of-principle demonstration of a technique that can probe the boundary of quantum mechanics and general relativity and perhaps offer the opportunity to reconcile the two experimentally. </jats:p>

<jats:title>Hot on the Trail</jats:title> <jats:p> Solar cells essentially operate by absorbing light, which needs to be above a certain energy threshold. The absorbed light then liberates charges within the solar cell to carry electrical current. Unfortunately, the liberated charges behave the same way whether they are excited right at the threshold (e.g., by visible light) or well above it (by ultraviolet light), which leads to any excess energy being dissipated as waste heat. <jats:bold> Tisdale <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1543" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185509">1543</jats:related-article> ) have documented a potential first step toward resolving this inefficiency. Specifically, electrons excited by light absorption in lead selenide nanocrystals were able to migrate to an adjacent titanium dioxide surface without releasing their excess energy to heat. The next step will be to devise a means of harnessing the stored energy in a circuit. </jats:p>