Springtime Warming Research Articles

Winter leaf reddening in ‘evergreen’ species

Leaf reddening during autumn in senescing, deciduous tree species has received widespread attention from the public and in the scientific literature, whereas leaf reddening in evergreen species during winter remains largely ignored. Winter reddening can be observed in evergreen herbs, shrubs, vines and trees in Mediterranean, temperate, alpine, and arctic regions, and can persist for several months before dissipating with springtime warming. Yet, little is known about the functional significance of this colour change, or why it occurs in some species but not others. Here, the biochemistry, physiology and ecology associated with winter leaf reddening are reviewed, with special focus on its possible adaptive function. Photoprotection is currently the favoured hypothesis for winter reddening, but alternative explanations have scarcely been explored. Intraspecific reddening generally increases with sunlight incidence, and may also accompany photosynthetic inferiority in photosynthetically 'weak' (e.g. low-nitrogen) individuals. Red leaves tend to show symptoms of shade acclimation relative to green, consistent with a photoprotective function. However, winter-red and winter-green species often cohabitate the same high-light environments, and exhibit similar photosynthetic capacities. The factors dictating interspecific winter leaf colouration therefore remain unclear. Additional outstanding questions and future directions are also highlighted, and possible alternative functions of winter reddening discussed.

Springtime warming and reduced snow cover from carbonaceous particles

Abstract. Boreal spring climate is uniquely susceptible to solar warming mechanisms because it has expansive snow cover and receives relatively strong insolation. Carbonaceous particles can influence snow coverage by warming the atmosphere, reducing surface-incident solar energy (dimming), and reducing snow reflectance after deposition (darkening). We apply a range of models and observations to explore impacts of these processes on springtime climate, drawing several conclusions: 1) Nearly all atmospheric particles (those with visible-band single-scatter albedo less than 0.999), including all mixtures of black carbon (BC) and organic matter (OM), increase net solar heating of the atmosphere-snow column. 2) Darkening caused by small concentrations of particles within snow exceeds the loss of absorbed energy from concurrent dimming, thus increasing solar heating of snowpack as well (positive net surface forcing). Over global snow, we estimate 6-fold greater surface forcing from darkening than dimming, caused by BC+OM. 3) Equilibrium climate experiments suggest that fossil fuel and biofuel emissions of BC+OM induce 95% as much springtime snow cover loss over Eurasia as anthropogenic carbon dioxide, a consequence of strong snow-albedo feedback and large BC+OM emissions from Asia. 4) Of 22 climate models contributing to the IPCC Fourth Assessment Report, 21 underpredict the rapid warming (0.64°C decade−1) observed over springtime Eurasia since 1979. Darkening from natural and anthropogenic sources of BC and mineral dust exerts 3-fold greater forcing on springtime snow over Eurasia (3.9 W m−2) than North America (1.2 W m−2). Inclusion of this forcing significantly improves simulated continental warming trends, but does not reconcile the low bias in rate of Eurasian spring snow cover decline exhibited by all models, likely because BC deposition trends are negative or near-neutral over much of Eurasia. Improved Eurasian warming may therefore relate more to darkening-induced reduction in mean snow cover.

Increase in vegetation greenness and decrease in springtime warming over east Asia

This study investigates the impact of increased vegetation greening on the springtime temperature over east Asia for 1982–2000. An analysis of station‐based temperature records and satellite‐measure normalized difference vegetation index (NDVI) indicates that slight warming (<0.4°C 10‐yr−1) occurred over regions that experienced large increase in NDVI (≥0.08 10‐yr−1). On the contrary, strong warming (≥0.8°C 10‐yr−1) occurred over regions that exhibited minor changes in NDVI (<0.04 10‐yr−1). For the most part, this inverse NDVI–temperature relationship observed with the daily maximum temperature. Thus, it is suggested that the decrease in warming was mostly attributable to the increase in evapotranspiration associated with increased vegetation greening. Earlier vegetation growth may have further strengthened the effect of this vegetation–evaporation on spring temperature.

A Multidecadal Trend of Earlier Corn Planting in the Central USA

The scientific literature suggests that a trend toward earlier corn (Zea mays L.) planting has taken place over the past several decades and is largely attributed to improving technology. To substantiate this idea, analysis of two datasets were performed: (i) U.S. Department of Agriculture (USDA) weekly crop progress data from 1979 to 2005 were analyzed to quantify trends in the date that 10, 25, 50, and 75% of corn planting had been completed across 12 Corn Belt states; (ii) National Centers for Environmental Prediction (NCEP) daily climate data were studied to understand whether springtime warming was a major contributor to any observed planting date trends. Statistical analysis suggested that the initiation of corn planting (10% planted) at the state level has become earlier (P < 0.05) by 0.3 to 0.8 d yr−1, with a regional, weighted average of 0.48 d yr−1 earlier (P < 0.0001). The regional average planting date trends were −0.45, −0.39, and −0.37 d yr−1 for the 25, 50, and 75% planted thresholds, respectively, and all were significant (P < 0.05). Consequently, the initiation of corn planting is now averaging approximately 2 wk earlier relative to the early 1980s. Continued development of genotypes that are tolerant of suboptimal temperatures, planting equipment improvements, and adoption of time‐saving management practices like conservation tillage are the more likely contributors to earlier planting rather than wide‐ranging springtime warming. However, it is unrealistic to expect that this long‐term trend will continue indefinitely because early planting will ultimately be limited by frozen soils.

Responses of secondary chemicals in sugar maple (Acer saccharum) seedlings to UV-B, springtime warming and nitrogen additions

Anticipated effects of climate change involve complex interactions in the field. To assess the effects of springtime warming, ambient ultraviolet-B radiation (UV-B) and nitrogen fertilization on the foliar chemistry and herbivore activity of native sugar maple (Acer saccharum Marsh.) seedlings, we carried out a field experiment for 2 years at two sugar maple forests growing on soils of contrasting acidity. At the Oliver site, soils are derived from a strongly calcareous till, whereas the naturally acidic soils and base-poor soils of the Haliburton site are derived from the largely granitic Precambrian Shield. At both sites, removal of ambient UV-B led to increases in chlorogenic acid and some flavonoids and reduced herbivore activity. At Haliburton, ammonium nitrate fertilization led to further increases in foliar manganese (Mn), whereas at Oliver there were no such changes. Nitrogen additions led to decreases in the concentrations of some flavonoids at both sites, but seedlings at Oliver had significantly higher concentrations of flavonoids and chlorogenic acid than seedlings at Haliburton. We suggest that this could be associated with increased mobilization of Mn due to increased soil acidity, which interferes with the role of calcium (Ca) in the phenolic biosynthetic pathway. It appears that the composition of the forest soil governs the response of seedlings when they are exposed to abiotic stressors.

The effects of UV-B, nitrogen fertilization, and springtime warming on sugar maple seedlings and the soil chemistry of two central Ontario forests

The interactive effects of springtime warming, ambient UV-B, and nitrogen fertilization on the chemistry of sugar maple (Acer saccharum Marsh.) seedlings and soils from two contrasting sites were assessed. Open-top chambers increased average springtime air temperatures by approximately 1.5 °C, but their heating effect was diminished upon closure of the overstory canopy. Ambient levels of UV-B were reduced with Mylar D polyester film. Ammonium nitrate fertilizer was added in an amount equivalent to an additional 50 kg N·ha–1. The soils of the Oliver forest were deep luvisols overlying a strongly calcareous till (average pH 6.0), while the naturally acidic soils of Haliburton were derived from the Precambrian Shield (average pH 4.7). Of the three main treatments used in this study, application of nitrogen fertilizer had the greatest impacts on foliar chemistry. At both sites, fertilizer application increased the acidity of the soils, while at Haliburton there were losses in total soil calcium. Haliburton maple seedlings had increased foliar concentrations of aluminum and manganese, decreased concentrations of calcium, and reduced calcium/manganese and magnesium/manganese nutrient ratios, after fertilizer was applied. Meanwhile, seedlings growing on the more alkaline soils of Oliver had increased foliar concentrations of magnesium following application of the nitrogen fertilizer. We suggest that these changes in the elemental chemistry of the soils and foliage brought on by continued nitrogen loading may predispose seedlings growing on naturally acidic soils, such as those of the Precambrian Shield, to further stress from additional abiotic and biotic stressors.

Changes toward Earlier Streamflow Timing across Western North America

Abstract The highly variable timing of streamflow in snowmelt-dominated basins across western North America is an important consequence, and indicator, of climate fluctuations. Changes in the timing of snowmelt-derived streamflow from 1948 to 2002 were investigated in a network of 302 western North America gauges by examining the center of mass for flow, spring pulse onset dates, and seasonal fractional flows through trend and principal component analyses. Statistical analysis of the streamflow timing measures with Pacific climate indicators identified local and key large-scale processes that govern the regionally coherent parts of the changes and their relative importance. Widespread and regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place across most of western North America, affecting an area that is much larger than previously recognized. These timing changes have resulted in increasing fractions of annual flow occurring earlier in the water year by 1–4 weeks. The immediate (or proximal) forcings for the spatially coherent parts of the year-to-year fluctuations and longer-term trends of streamflow timing have been higher winter and spring temperatures. Although these temperature changes are partly controlled by the decadal-scale Pacific climate mode [Pacific decadal oscillation (PDO)], a separate and significant part of the variance is associated with a springtime warming trend that spans the PDO phases.

Simulating Springtime Temperature Patterns in the Community Atmosphere Model Coupled to the Community Land Model Using Prognostic Leaf Area

Abstract Observations show that emergence of foliage in springtime slows surface air temperature warming as a result of greater transpiration. Model simulations with the Community Atmosphere Model coupled to the Community Land Model confirm that evapotranspiration contributes to this pattern and that this pattern occurs more reliably with prognostic leaf area as opposed to prescribed leaf area. With prescribed leaf area, leaves emerge independent of prevailing environmental conditions, which may preclude photosynthesis from occurring. In contrast, prognostic leaf area ensures that leaves emerge when conditions are favorable for photosynthesis, and thus transpiration. These results reveal a dynamic coupling between the atmosphere and vegetation in which the observed reduction in the springtime warming trend only occurs when photosynthesis, stomatal conductance, and leaf emergence are synchronized with the surface climate.

Recent temperature and precipitation increases in West Siberia and their association with the Arctic Oscillation

Surface air temperature and precipitation records for the years 1958-1999 from ten meteorological stations located throughout West Siberia are used to identify climatic trends and determine to what extent these trends are potentially attributable to the Arctic Oscillation (AO). Although recent changes in atmospheric variability are associated with broad Arctic climate change, West Siberia appears particularly susceptible to warming. Furthermore, unlike most of the Arctic, moisture transport in the region is highly variable. The records show that West Siberia is experiencing significant warming and notable increases in precipitation, likely driven, in part, by large-scale Arctic atmospheric variability. Because this region contains a large percentage of the world's peatlands and contributes a significant portion of the total terrestrial freshwater flux to the Arctic Ocean, these recent climatic trends may have globally significant repercussions. The most robust patterns found are strong and prevalent springtime warming, winter precipitation increases, and strong association of non-summer air temperatures with the AO. Warming rates for both spring (0.5-0.8 °C/decade) and annual (0.3-0.5°C/decade) records are statistically significant for nine often stations. On average, the AO is linearly congruent with 96% (winter), 19% (spring), 0% (summer), 67% (autumn) and 53% (annual) of the warming found in this study. Significant trends in precipitation occur most commonly during winter, when four of ten stations exhibit significant increases (4-13 %/decade). The AO may play a lesser role in precipitation variability and is linearly congruent with only 17% (winter), 13% (spring), 12% (summer), 1% (autumn) and 26% (annual) of precipitation trends.

Possible Ozone-Induced Long-Term Changes in Planetary Wave Activity in Late Winter

Using NCEP–NCAR reanalysis data, decadal trends in planetary wave activity in Northern Hemisphere high latitudes (50°–90°N) in late winter and early spring (January–February–March) were studied. Results show that wave activity in both the stratosphere and the troposphere has been largely reduced and exhibits statistically significant downward trends since the 1980s. In the stratosphere, the wave activity is decreased by about 30%, which is mainly due to less Eliassen–Palm (E–P) flux from the troposphere into the stratosphere. In the troposphere, the vertical E–P flux is reduced by about 30%, while equatorward horizontal E–P flux is increased by 130%. This suggests a significant refraction of planetary waves away from the high latitudes. The significant negative trends in wave activity in late winter are in contrast to the authors' previous finding of no significant changes in planetary wave activity in early winter. The timing of the significant decline in wave activity, which starts from the early 1980s and exists only in late winter and springtime, suggests that such a decrease of wave activity is possibly a result of stratospheric ozone depletion in the Arctic. Therefore, a mechanism is proposed whereby Arctic ozone depletion leads to an enhanced meridional temperature gradient near the subpolar stratosphere, strengthening westerly winds. The strengthened winds refract planetary waves toward low latitudes and cause the reduction in wave activity in high latitudes. Decreasing vertical E–P fluxes are found to extend to near the surface. At 850 mb, vertical E–P fluxes have been reduced by about 10% since 1979. Such a reduction in wave activity might be responsible for the observed late-winter and springtime warming over Northern Hemisphere high-latitude continents during the last two decades.

Recent temperature and precipitation increases in West Siberia and their association with the Arctic Oscillation

Surface air temperature and precipitation records for the years 1958-1999 from ten meteorological stations located throughout West Siberia are used to identify climatic trends and determine to what extent these trends are potentially attributable to the Arctic Oscillation (AO). Although recent changes in atmospheric variability are associated with broad Arctic climate change, West Siberia appears particularly susceptible to warming. Furthermore, unlike most of the Arctic, moisture transport in the region is highly variable. The records show that West Siberia is experiencing significant warming and notable increases in precipitation, likely driven, in part, by large-scale Arctic atmospheric variability. Because this region contains a large percentage of the world's peatlands and contributes a significant portion of the total terrestrial freshwater flux to the Arctic Ocean, these recent climatic trends may have globally significant repercussions. The most robust patterns found are strong and prevalent springtime warming, winter precipitation increases, and strong association of non-summer air temperatures with the AO. Warming rates for both spring (0.5-0.8 °C/decade) and annual (0.3-0.5°C/decade) records are statistically significant for nine often stations. On average, the AO is linearly congruent with 96% (winter), 19% (spring), 0% (summer), 67% (autumn) and 53% (annual) of the warming found in this study. Significant trends in precipitation occur most commonly during winter, when four of ten stations exhibit significant increases (4-13 %/decade). The AO may play a lesser role in precipitation variability and is linearly congruent with only 17% (winter), 13% (spring), 12% (summer), 1% (autumn) and 26% (annual) of precipitation trends.

Interannual variations in satellite‐sensed vegetation index data from 1981 to 1991

Normalized difference vegetation index (NDVI) data processed from measurements of advanced very high resolution radiometers (AVHRR) onboard the afternoon‐viewing NOAA series satellites (NOAA 7, 9, and 11) were analyzed for spatial and temporal patterns comparable to those observed in atmospheric CO2, near‐surface air temperature, and sea surface temperature (SST) data during the 1981–1991 time period. Two global data sets of NDVI were analyzed for consistency: (1) the land segment of the joint NOAA/NASA Earth Observing System AVHRR Pathfinder data set and (2) the Global Inventory Monitoring and Modeling Studies AVHRR NDVI data set. The impact of SST events was found to be confined mostly to the tropical latitudes but was generally dominant enough to be manifest in the global NDVI anomaly. The vegetation index anomalies at latitudes north of 45°N were found to exhibit an increasing trend. This linear trend corresponds to a 10% increase in seasonal NDVI amplitude over a 9 year period (1981–1990). During the same time period, annual amplitude in the record of atmosphere CO2 measured at Point Barrow, Alaska, was reported to have increased by about 14%. The increase in vegetation index data between years was especially consistent through the spring and early summer time periods. When this increase was translated into an advance in the timing of spring green‐up, the measure (8±3 days) was similar to the recently published estimate of about 7 days in the advance of the midpoint of CO2 drawdown between spring and summer at Point Barrow, Alaska. The geographical distribution of the increase in vegetation activity was consistent with the reported patterns in springtime warming and decline of snow cover extent over the northern hemisphere land area.

Predicted reduction in basal melt rates of an Antarctic ice shelf in a warmer climate

Floating ice shelves are vulnerable to climate change at both their upper and lower surfaces. The extent to which the apparently air-temperature-related retreat of some northerly Antarctic Peninsula ice shelves1 presages the demise of their much larger, more southerly, counterparts is not known, but air-temperature effects are unlikely to be important in the near future. Oceanographic measurements from beneath the most massive of these southerly ice shelves—the Filchner–Ronne Ice Shelf2,3,4—have confirmed that dense sea water resulting from sea-ice formation north of the ice shelf flows into the sub-ice-shelf cavity. This relatively warm so-called High Salinity Shelf Water (HSSW) is responsible for the net melting at the ice shelf's base. Here I present temperature measurements, from the same sub-ice-shelf cavity, which show a strong seasonality in the inflow of HSSW. This seasonality results from intense wintertime production of sea ice, and I argue that the seasonal springtime warming can be used as an analogue for climate warming. For the present mode of oceanographic circulation, the implication is that warmer winters (a climate warming, leading to lower rates of sea-ice formation, would cause a reduction in the flux of HSSW beneath the ice shelf. The resultant cooling in the sub-ice cavity would lead, in turn, to a reduction in the total melting at the ice shelf's base. A moderate warming of the climate could thus lead to a basal thickening of the Filchner–Ronne Ice Shelf, perhaps increasing its longevity.

Two‐dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with Halogen Occultation Experiment data

A two‐dimensional chemical transport model has been used to examine the physical processes governing the transport of high levels of thermospheric nitric oxide (NO) downward into the middle atmosphere. Three different facets of this transport are studied. The first facet involves diffusion from the thermosphere to the summertime mesopause region. The second facet involves downward advection by the mean meridional circulation in the wintertime mesosphere and the effects of planetary wave mixing on the latitudinal gradient of NO. The third facet is the residual amount of NO deposited in the springtime upper stratosphere and its senstivity to the magnitude and duration of the unmixed descent which occurred the previous winter. Comparison of the model with observations by the Halogen Occultation Experiment (HALOE) suggest the following: (1) A clear auroral enhancement in summertime NO exists at 89 km. Model calculations suggest this results from both in situ ionization and dissociation of N2 as well as downward diffusion from the thermosphere above 100 km. (2) Using HALOE CH4 observations as a tracer, enhanced NO in the wintertime mesosphere is seen to be transported to latitudes as far equatorward as 30°–40°. The model is in good agreement with these observations when planetary wave mixing is included. Without this mixing, the enhanced NO remains confined to high latitudes that are not observed by HALOE in winter. (3) The model overestimates the net NO deposited into the upper stratosphere. This appears to be related to the model springtime warming being delayed relative to the real atmosphere. Inclusion of an additional source of drag in the polar stratosphere in late winter yields better agreement with observations.

Modelling of the generation and evolution of the cold intermediate layer in the Black Sea

This paper discusses the results of a numerical experiment on modelling the seasonal variability of the water circulation and temperature/salinity fields in the Black Sea. A multi-layered quasi-isopycnic model is used based on primitive equations and incorporating the upper mixed layer (UML). It is shown that during springtime warming, relatively cold layers emerge in some areas, due to subduction, from the UML, which then spread virtually over the entire basin and persist until they become absorbed by the rapidly deepening UML during the period of cooling in winter.

Abundance, distribution and population structure of the copepod Calanus finmarchicus in a springtime right whale feeding area in the southwestern Gulf of Maine

Springtime aggregations of the planktivorous right whale ( Eubalaena glacialis) occur in the northern Great South Channel region of the southwestern Gulf of Maine, where they feed upon dense concentrations of the copepod Calanus finmarchicus. This association was studied during the multidisciplinary South Channel Ocean Productivity Experiment (SCOPEX) in 1988 and 1989. The spatial and temporal variability of the abundance, geographic distribution, and population structure of these copepods were analyzed using data from 99 vertically-stratified or horizontally-sequenced MOCNESS plankton tows. Higher water column abundances and higher relative proportion of older copepod lifestages occurred near feeding whales compared to sites without whales, but total water column copepod biomass and Calanus abundance did not always differ between these types of locations. This suggests that the whales seek out aggregations of older copepod lifestages rather than simply the most dense aggregations. Other factors (and perhaps an element of chance) may influence which specific patches, among all patches potentially suitable in terms of copepod abundance and age composition, the whales utilize at a particular time. The times and locations of the highest Calansus water column abundances varied between years, as did the presence of feeding whales, probably because of year-to-year differences in the springtime temperature cycle and current strength. A temporal progression of lifestages occurred within the region in both years during the roughly 3-week duration of each survey, indicative of a growing rather than a diapausing population, at least up to the copepodite 4 (C4) stage. Due in part to a delay in the springtime warming in 1989 compared to 1988, the copepod development cycle, which is largely driven by in situ temperature, was delayed about 1–2 weeks in 1989. Peak abundances of younger Calanus were found in the northwestern part of the region each year, whereas peak abundances of older Calanus were found in the southwestern and northeastern part. This was probably due to the advection of maturing copepods by the regional circulation, especially the near-surface current associated with the movement of the low-salinity surface plume which forms each spring off Cape Cod. The copepod development cycle occurs within a moving frame of reference (i.e. the water itself); thus, peak abundances of the older copepods (those fed on by the whales) occurred later in the spring and further downstream in 1989 (when there were colder springtime temperatures and faster currents) than in 1988 (when the springtime temperatures were warmer and currents slower). Maximum Calanus abundances and biomass and water-colum abundances of older copepodite stages were significantly higher (about double) in 1989 than in 1988, both in the region as a whole and at sites where whales were feeding. Maximum concentrations from the MOCNESS tows were 13,300 m −3 in 1988 and 30,800 m −3 in 1989; however, a thin, visibly-red surface patch of Calanus, sampled in 1989 by a bucket, had a concentration of 331,000 m −3. Copepods were also more aggregated in the vertical (i.e. more highly concentrated at the depth of maximum abundance) in 1989 than in 1988, and samples from whale-feeding areas were more homogeneous in composition (higher proportion of Calanus relative to all zooplankton) in 1989. At smaller spatial and temporal scales, abundances varied by a factor of 1–890 X in samples from horizontal tows spanning about 0.5–1.5 km and by a factor of 1–50 X over 24 h in the same geographic location in whale-feeding areas. Some of this variability was probably due to advection by the semidiurnal tidal currents. Near feeding whales, the copepod spatial distribution was patchy on small scales (with an estimated mean patch “size” of about 500 m), but the patchiness varied in texture interannually. Copepod abundances were much lower in early spring (March 1988) than in later spring (May 1988), with the March population structure dominated by adult females and the May population dominated by copepodite 4 and 5 stages (C4 and C5).