Northern Hemisphere Land Masses Research Articles

Using the current seasonal cycle to constrain snow albedo feedback in future climate change

Differences in simulations of climate feedbacks are sources of significant divergence in climate models' temperature response to anthropogenic forcing. Snow albedo feedback is particularly critical for climate change prediction in heavily‐populated northern hemisphere land masses. Here we show its strength in current models exhibits a factor‐of‐three spread. These large intermodel variations in feedback strength in climate change are nearly perfectly correlated with comparably large intermodel variations in feedback strength in the context of the seasonal cycle. Moreover, the feedback strength in the real seasonal cycle can be measured and compared to simulated values. These mostly fall outside the range of the observed estimate, suggesting many models have an unrealistic snow albedo feedback in the seasonal cycle context. Because of the tight correlation between simulated feedback strength in the seasonal cycle and climate change, eliminating the model errors in the seasonal cycle will lead directly to a reduction in the spread of feedback strength in climate change. Though this comparison to observations may put the models in an unduly harsh light because of uncertainties in the observed estimate that are difficult to quantify, our results map out a clear strategy for targeted observation of the seasonal cycle to reduce divergence in simulations of climate sensitivity.

Climatic response to high‐latitude volcanic eruptions

Strong volcanic eruptions can inject large amounts of SO2 into the lower stratosphere, which over time, are converted into sulfate aerosols and have the potential to impact climate. Aerosols from tropical volcanic eruptions like the 1991 Mount Pinatubo eruption spread over the entire globe, whereas high‐latitude eruptions typically have aerosols which remain in the hemisphere in which they where injected. This causes their largest radiative forcing to be extratropical, and the climate response should be different from that of tropical eruptions. We conducted a 20‐member ensemble simulation of the climate response to the Katmai eruption (58°N) of 6 June 1912 using the NASA Goddard Institute for Space Studies ModelE climate model. We also produced an additional 20‐member ensemble for a 3 times Katmai (3x Katmai) eruption to see the impact the strength of the eruption has on the radiative as well as the dynamical responses. The results of these simulations do not show a positive Arctic Oscillation response like past simulations of tropical volcanic eruptions, but we did find significant cooling over southern Asia during the boreal winter. The first winter following Katmai and the second winter following 3x Katmai showed strong similarities in lower stratospheric geopotential height anomalies and sea level pressure anomalies, which occurred when the two cases had similar optical depth perturbations. These simulations show that the radiative impact of a high‐latitude volcanic eruption was much larger than the dynamical impact at high latitudes. In the boreal summer, however, strong cooling over the Northern Hemisphere landmasses caused a decrease in the Asian monsoon circulation with significant decreases of up to 10% in cloud cover and warming over northern India. Thus the main dynamical impact of high‐latitude eruptions is in the summer over Asia.

Relative Importance of Northern Hemisphere Circulation Modes in Predicting Regional Climate Change

Abstract The Northern Hemisphere annular mode (NAM), North Atlantic Oscillation (NAO), and Aleutian low (AL) are known to be the most prominent components of Northern Hemisphere (NH) near-surface climate variability. In a tremendous number of studies, the impact of these circulation features on regional climate has been demonstrated. More recently, research has gone into the connection between the NAO and NAM and into the physical meaning of the latter. However, the relevance of those circulation modes for climatological issues may also be inferred from another nondynamical point of view: their statistical relationship to various climate parameters. This study comprises two steps: 1) qualifying and quantifying the relative importance of NH circulation modes with respect to twentieth-century near-surface temperature and precipitation, using stepwise multiple regression with cross validation; and 2) using predictor–predictand relationships to access the contributions of each circulation mode to regional climate change in the middle of the twenty-first century, given multimodel predictions of the circulation modes' responses to increasing greenhouse gas (GHG) and sulfate aerosol (SUL) concentrations. Altogether, the NAM, NAO, and AL account locally for up to 75% of the total interannual temperature and rainfall variability over NH continents. Over the major part of the NH, the NAM appears to be the most important predictor. In some parts of the North Atlantic, temperature and rainfall are more closely linked to the NAO, while the North Pacific is clearly dominated by the AL dynamics. In general, the NAO and AL have a more regionally confined influence. Climate change experiments mostly predict an intensification of the NAM and AL under GHG+SUL forcing, while the NAO response is much less consistent with different models and generally undergoes no long-term changes. This leads to substantial contributions to temperature and rainfall anomalies, especially over the NH landmasses. Temperature changes amount to ±1 K over large parts of Russia, North America, and the North Pacific. The major precipitation changes occur over the North Pacific, the North Atlantic, and Scandinavia. This circulation-induced contribution accounts for a considerable part of total expected change in these regions. Given its distinct trend, the NAM plays the main role, except over the Pacific Ocean and North America, where the AL is driving regional climate anomalies. Thus, whether physically relevant or not, the NAM is an appropriate statistical indicator of NH regional climate change.

A three‐dimensional model of molecular hydrogen in the troposphere

The global distribution and budget of atmospheric molecular hydrogen (H2) is simulated with a global Chemistry‐Transport Model (CTM). Surface emissions include technological sources (industry, transportation and other fossil fuel combustion processes), biomass burning, nitrogen fixation in soils, and oceanic activity and totals 39 Tg/yr. The photochemical production (31 Tg/yr) from formaldehyde photolysis accounts for about 45% of the total source of H2. Soil uptake (55 Tg/yr) represents a major loss process for H2 and contributes for 80% to the total destruction. H2 oxidation by OH in the troposphere contributes the remainder. The global burden of H2 in the atmosphere is 136 Tg. Its overall lifetime in the atmosphere is 1.9 years. H2 is rather well‐mixed in the free troposphere. However, its distribution shows a significant seasonal variation in the lower troposphere where soil uptake dominates. This loss process shows a strong temporal variability and is maximum over the northern hemisphere landmass during summer. Strong vertical gradients result from this surface uptake. In these regions, H2 varies by more than 30% between the maximum mixing ratio in winter and the summer minimum. Our results stress the important role played by the tropics in the budget of H2. In these regions a strong seasonal cycle is also predicted due to the annual variation in biomass burning emissions, soil uptake, and rapid transport by convection of H2 depleted air masses from the boundary layer to the upper troposphere. A comparison with the observed H2 distribution allows to test some of the model predictions. Good agreement is found for the global burden and the annually averaged latitudinal gradient in the southern hemisphere and the tropics. A detailed comparison of the seasonal cycles of H2 in surface air indicates that the use of the net primary productivity to prescribe the seasonal and geographical pattern of soil uptake in the model leads to an underestimate of the deposition velocity during winter and spring over the continents in the northern hemisphere.

Simulation of recent northern winter climate trends by greenhouse-gas forcing

The temperature of air at the Earth's surface has risen during the past century1, but the fraction of the warming that can be attributed to anthropogenic greenhouse gases remains controversial. The strongest warming trends have been over Northern Hemisphere land masses during winter, and are closely related to changes in atmospheric circulation. These circulation changes are manifested by a gradual reduction in high-latitude sea-level pressure, and an increase in mid-latitude sea-level pressure associated with one phase of the Arctic Oscillation (a hemisphere-scale version of the North Atlantic Oscillation)2. Here we use several different climate-model versions to demonstrate that the observed sea-level-pressure trends, including their magnitude, can be simulated by realistic increases in greenhouse-gas concentrations. Thus, although the warming appears through a naturally occurring mode of atmospheric variability, it may be anthropogenically induced and may continue to rise. The Arctic Oscillation trend is captured only in climate models that include a realistic representation of the stratosphere, while changes in ozone concentrations are not necessary to simulate the observed climate trends. The proper representation of stratospheric dynamics appears to be important to the attribution of climate change, at least on a broad regional scale.

On Asian ornithopods (Dinosauria: Ornithischia). 3. A new species of iguanodontid dinosaur

The holotype of the Mongolian species of ornithopod dinosaurIguanodon orientalisfrom the Early Cretaceous (Barremian-Aptian of Khamarin Khural) has been shown to have been established on holotype material that is non-diagnostic, but appears to be closely similar osteologically to the Western European speciesI. bernissartensis. Additional material collected from the locality known as Khuren Dukh (Dornogov′, Mongolia), which had previously been referred toI. orientalishas been re-examined and shown to represent a new genus and species (Altirhinus kurzanovigen.etsp. nov) of ornithopod dinosaur from the Early Cretaceous (Late Aptian/Early Albian). The anatomy of this new ornithopod is described; it shows a number of similarities to the known species ofIguanodon, but also demonstrates features which are in some instances unique to this taxon, while others seem to be either convergent upon, or transformational with respect to, the more derived hadrosaurid ornithopods of the Late Cretaceous. Palaeogeographic evidence is corroborative in that it suggests contemporaneous, albeit episodic, links between the Northern Hemisphere landmasses during the Barremian-Albian interval; these explain the appearance of very ‘European’ large ornithopods (Iguanodon) in Asia in Barremian/Aptian times as a consequence of land-based dispersal. Subsequent isolation of Asia from the European ‘domain’ during the late Early Cretaceous (Albian) may be responsible for the appearance of derived forms such asAltirhinus, and is suggestive of an Asian centre of origin for the family Hadrosauridae in middle Cretaceous times; this contradicts an earlier vicariance-biogeographic model of ornithopod evolution. The cranial anatomical modifications seen in this new taxon: vertical expansion of the dorsal nasal cavity, lateral expansion and lowering of the cropping beak relative to the jaw line, increase in the number of replacement teeth (but no significant miniaturization of the crowns) and the trend toward formation of a more integrated battery of cheek teeth, seen to varying degrees in several mid-Cretaceous ornithopods are commented upon; they can be interpreted within the context of an evolutionary trend culminating in the cranial complexity seen in the terminal lineage of ornithopods represented by the Late Cretaceous Hadrosauridae. Functionally, some of these changes can be correlated with what can be interpreted as ‘improvements’ to the efficiency of food gathering and processing which might represent increased niche partitioning and/or responses to increasingly tough and abrasive (xeric adapted) foliage; others, notably the modifications to the nasal cavity (perhaps associated with providing space for a counter-current moisture conserving turbinal system), are suggestive of a biological response to increasingly seasonal/xeric conditions in the middle of the Cretaceous Period or changes in the floral composition of these times.

On Asian ornithopods (Dinosauria: Ornithischia). 3. A new species of iguanodontid dinosaur

The holotype of the Mongolian species of ornithopod dinosaurIguanodon orientalisfrom the Early Cretaceous (Barremian-Aptian of Khamarin Khural) has been shown to have been established on holotype material that is non-diagnostic, but appears to be closely similar osteologically to the Western European speciesI. bernissartensis. Additional material collected from the locality known as Khuren Dukh (Dornogov′, Mongolia), which had previously been referred toI. orientalishas been re-examined and shown to represent a new genus and species (Altirhinus kurzanovigen.etsp. nov) of ornithopod dinosaur from the Early Cretaceous (Late Aptian/Early Albian). The anatomy of this new ornithopod is described; it shows a number of similarities to the known species ofIguanodon, but also demonstrates features which are in some instances unique to this taxon, while others seem to be either convergent upon, or transformational with respect to, the more derived hadrosaurid ornithopods of the Late Cretaceous. Palaeogeographic evidence is corroborative in that it suggests contemporaneous, albeit episodic, links between the Northern Hemisphere landmasses during the Barremian-Albian interval; these explain the appearance of very ‘European’ large ornithopods (Iguanodon) in Asia in Barremian/Aptian times as a consequence of land-based dispersal. Subsequent isolation of Asia from the European ‘domain’ during the late Early Cretaceous (Albian) may be responsible for the appearance of derived forms such asAltirhinus, and is suggestive of an Asian centre of origin for the family Hadrosauridae in middle Cretaceous times; this contradicts an earlier vicariance-biogeographic model of ornithopod evolution. The cranial anatomical modifications seen in this new taxon: vertical expansion of the dorsal nasal cavity, lateral expansion and lowering of the cropping beak relative to the jaw line, increase in the number of replacement teeth (but no significant miniaturization of the crowns) and the trend toward formation of a more integrated battery of cheek teeth, seen to varying degrees in several mid-Cretaceous ornithopods are commented upon; they can be interpreted within the context of an evolutionary trend culminating in the cranial complexity seen in the terminal lineage of ornithopods represented by the Late Cretaceous Hadrosauridae. Functionally, some of these changes can be correlated with what can be interpreted as ‘improvements’ to the efficiency of food gathering and processing which might represent increased niche partitioning and/or responses to increasingly tough and abrasive (xeric adapted) foliage; others, notably the modifications to the nasal cavity (perhaps associated with providing space for a counter-current moisture conserving turbinal system), are suggestive of a biological response to increasingly seasonal/xeric conditions in the middle of the Cretaceous Period or changes in the floral composition of these times.

North American and Eurasian snow cover co-variability

The extent of snow cover over the Northern Hemisphere landmasses was computed over the period January 1973 to May 1996 using monthly data derived from the NOAA/NESDIS data set. A comparison has been made of the variability over the North American and Eurasian continents. We have found there to be significant memory in anomalies of snow cover for up to 3 months. We have also identified significant temporal associations between snow variability over North America and Eurasia, and these tended to be larger when North America was leading. Some suggestions for the link in variability are made. We have made a similar calculation using passive microwave sea-ice data from January 1979 to July 1995 in order to determine whether there is any link between the variability of sea-ice and that of continental snow cover. Over the period we have examined, there appeared to be a moderate connection between North American snow and Arctic sea-ice but no connection for Eurasia.

Modelled atmospheric response to changes in Northern Hemisphere snow cover

The surface boundary conditions are altered in a numerical simulation of January climate by prescribing (a) higher and (b) lower than average snow extent over Northern Hemisphere land masses. The anomalies in snow cover are shown to have quite a strong impact on the mean climatic state. Associated with an increase in the areal extent of the snow, there is a significant reduction in temperature throughout the lower troposphere. There are also large increases in sea-level pressure over most land areas. Significant responses in the mass field are also seen at 500 hPa where reductions in atmospheric thickness lead to significant negative anomalies in the height field. Responses are also seen non-locally, over both the North Pacific and North Atlantic basins. The impact of increased snow on cyclone tracks is also examined. A reduction in cyclones is noted over both continents and over the western sectors of both ocean basins. Over the North Atlantic basin this reduction extends across over Europe, significantly weakening the storm track. In the North Pacific, cyclone density is reduced in the west while in the east, there is actually a strengthening of the storm tracks. There are corresponding changes in the genesis of cyclones in both of these regions. The change in cyclogenesis, intensity and density is demonstrated to be associated with changes in baroclinicity between the two experiments. The anomalous snow boundary conditions lead to significant changes in the meridional temperature gradients over both ocean basins which impact on the baroclinic zones.

The influence of night time cloud cover on the observed minimum temperature in China

It has been established that minimum temperatures have been increasing with time at a higher rate than maximum temperatures for the past several decades, most prominently for large parts of the Northern Hemisphere land mass, but in other areas as well. We demonstrate that (for China, at least) this increase in nighttime temperatures is not necessarily the result of changes in cloud cover for the period 1951–1990.

Recent variations and regional relationships in Northern Hemisphere snow cover

An analysis of snow-cover variability over Northern Hemisphere land masses reveals a continuation of the subnormal coverage that began in the late 1980s (relative to the 1972–present interval). While the 1994 snow year (September 1993–August 1994) exhibited a return to near-normal hemispheric extent, only three months during this period had above-normal coverage. Only 11 of the past 88 months (through October 1994) have been above the norm. Deficits have been most common in spring, over both the Eurasian and North American continents. This is a hemisphere-wide situation; positive correlations are identified between hemispheric and regional snow extents in spring, as well as in fall and winter. A number of significant associations are also recognized between regions during these three seasons; however, it is uncommon to see more than 50% of the variance in one region explained by another. These correlations are most common between adjacent regions, but some are found between regions on different continents. Only in spring are significant positive relationships between non-adjacent regions on the same continent observed.

Recent variations and regional relationships in Northern Hemisphere snow cover

An analysis of snow-cover variability over Northern Hemisphere land masses reveals a continuation of the subnormal coverage that began in the late 1980s (relative to the 1972–present interval). While the 1994 snow year (September 1993–August 1994) exhibited a return to near-normal hemispheric extent, only three months during this period had above-normal coverage. Only 11 of the past 88 months (through October 1994) have been above the norm. Deficits have been most common in spring, over both the Eurasian and North American continents. This is a hemisphere-wide situation; positive correlations are identified between hemispheric and regional snow extents in spring, as well as in fall and winter. A number of significant associations are also recognized between regions during these three seasons; however, it is uncommon to see more than 50% of the variance in one region explained by another. These correlations are most common between adjacent regions, but some are found between regions on different continents. Only in spring are significant positive relationships between non-adjacent regions on the same continent observed.

A New Perspective on Recent Global Warming: Asymmetric Trends of Daily Maximum and Minimum Temperature

Monthly mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951–90 (0.84°C versus 0.28°C). The decrease of the diurnal temperature range is approximately equal to the increase of mean temperature. The asymmetry is detectable in all seasons and in most of the regions studied. The decrease in the daily temperature range is partially related to increases in cloud cover. Furthermore, a large number of atmospheric and surface boundary conditions are shown to differentially affect the maximum and minimum temperature. Linkages of the observed changes in the diurnal temperature range to large-scale climate forcings, such as anthropogenic increases in sulfate aerosols, greenhouse gases, or biomass burning (smoke), remain tentative. Nonetheless, the observed decrease of the diurnal temperature range is clearly important, both scientifically and practically.

Southwest Pacific temperatures: Diurnal and seasonal trends

Temperature trends are presented for a large part of the southwest Pacific. The trends differ from those documented for Northern Hemisphere land areas, where warming has occurred mainly through increases in minimum temperature. The New Zealand patterns are derived from recently completed analyses of monthly and annual mean maximum and minimum surface temperature records for a newly homogenised historical climate data series for New Zealand and outlying islands. They indicate that the warming in the New Zealand region over the past five decades can be attributed to increases in both mean maximum (mostly daytime) and mean minimum (mostly night time) temperature. All seasons show a temperature increase, with the largest occurring in summer (DJF).Northern Hemisphere evidence suggests that changes in cloud cover and the presence of sulfate aerosols plays a direct role. The present results imply that, while the observed warming in a large portion of the Northern Hemisphere landmass may be significantly affected by both these factors, sulfate aerosol effects may be less important in the Southern Hemisphere.

Global warming: Evidence for asymmetric diurnal temperature change

Analyses of the year‐month mean maximum and minimum surface thermometric record have now been updated and expanded to cover three large countries in the Northern Hemisphere (the contiguous United States, the Soviet Union, and the People's Republic of China). They indicate that most of the warming which has occurred in these regions over the past four decades can be attributed to an increase of mean minimum (mostly nighttime) temperatures. Mean maximum (mostly daytime) temperatures display little or no warming. In the USA and the USSR (no access to data in China) similar characteristics are also reflected in the changes of extreme seasonal temperatures, e.g., increase of extreme minimum temperatures and little or no change in extreme maximum temperatures. The continuation of increasing minimum temperatures and little overall change of the maximum leads to a decrease of the mean (and extreme) temperature range, an important measure of climate variability.The cause(s) of the asymmetric diurnal changes are uncertain, but there is some evidence to suggest that changes in cloud cover plays a direct role (where increases in cloudiness result in reduced maximum and higher minimum temperatures). Regardless of the exact cause(s), these results imply that either: (1) climate model projections considering the expected change in the diurnal temperature range with increased levels of the greenhouse gases are underestimating (overestimating) the rise of the daily minimum (maximum) relative to the maximum (minimum), or (2) the observed warming in a considerable portion of the Northern Hemisphere landmass is significantly affected by factors unrelated to an enhanced anthropogenically‐induced greenhouse effect.

The effect of volcanic aerosols on global climate

The surface temperature decrease after major volcanic eruptions has been demonstrated by a number of groups to be the order of a few tenths of a degree. At first, the stratospheric aerosol is usually restricted to a narrow range of latitudes, and the volcanically induced temperature decrease is found to be nonuniform over the surface of the earth. In fact, because of the differential cooling of land and ocean there should always be volcanically induced temperature gradients. These temperature gradients will induce a redistribution of air mass and anomalous sea level pressure gradients, anomalous winds and anomalous climate on a global scale. In this paper, a hypothesis is presented which assumes that volcanic aerosols, by reducing the solar radiation, induces surface temperature gradients and sea level pressure gradients between the land and the oceans. Since cooling of the land occurs before the oceans, there will be, at first, an increase of sea level pressure over land relative to the oceans. Conservation of total air mass on a global scale requires the sea level pressure over the ocean to be reduced.Because the majority of the land area is in the Northern Hemisphere, the volcanically induced cooling will transfer air mass from the oceanic anticyclones in both the Northern and Southern Hemisphere to the continents and especially to southern Eurasia. The anomalous sea level pressure pattern (SLP) which would be obtained — higher SLP over southern Eurasia and lower SLP over the southern oceans — is very close to the pattern, first named by Sir Gilbert Walker, called the negative phase of the Southern Oscillation. The positive phase of the Southern Oscillation can occur when no volcanic aerosols are present and the stratosphere is clear. Then the additional radiation and heating of the land increases the transfer of air mass from the Northern Hemisphere land mass to the world's oceans (the positive phase of the global Southern Oscillation).In this paper, it will be shown that this hypothesis has the potential to explain the observed association between low-latitude volcanic aerosols and the El Niño/Southern Oscillation. It also has the potential to explain why El Chichón's eruption induced a very strong El Niño compared to the weaker El Niño events induced by the eruptions of Agung and Krakatau. It also explains why almost all monsoon countries experience below-normal precipitation just prior to the maximum of the El Niño warming. The hypothesis also has the potential to explain why low- and high-latitude aerosols will affect the global climate system differently.This hypothesis represents a return to the ideas of Sir Gilbert Walker, who first defined the Southern Oscillation as a global-scale anomaly rather than an index between just two stations.

The Effect of Urban Warming on the Northern Hemisphere Temperature Average

The significance of the urban warming effect on large-scale and hemispheric mean temperature series is assessed using estimates of the urbanization bias for stations in the United States produced by Karl et al. It is concluded that the Northern Hemisphere landmass average recently compiled by Jones et al. may contain a spurious warming trend which is, at the maximum, 0.1°C over the first eight decades of the twentieth century. This is considerably less than the long-term warming trend observed over the same period.

The late Ordovician—Early Silurian glacial period

Late Ordovician to Early Silurian tillites and evidence of extensive glacial erosion were first discovered in the western Sahara in the late 1950's, although the concept of a glacial period at this time had been around since the late 19th century. Further evidence of glaciation is now available from West Africa, South Africa, the Arabian Peninsula, eastern North America, west and central Southe America and Europe. Several glacial episodes can be distinguished. The earliest evidence (local mountain glaciation?) appears to be of Caradoc age. The major episode may have begun in Caradoc time and lasted much of Ashgill time, when a major ice-sheet covered at least North-Central and West Africa which was then centered near the South Pole. However, independent palaeontological and sea-level evidence from areas far removed from the centre of glaciation suggests a time span for this episode of less than 2 Ma. This conflict has yet to be resolved. The latest evidence of glaciation is Wenlock in age but is confined to South America, which by then was replacing Africa over the South Polar region. However, this latest episode may simply have been a rather intensive mountain glaciation rather than being of ice-sheet dimensions. The lack of land in the North Polar region probably explains the absence of tillites from the then northern hemisphere land masses. The total area affected by glaciation in Early Palaeozoic time in North Africa alone is at least 6 × 10 6 km 2. Combined with other glaciated areas glaciation may have been on the scale of that of present-day Antarctica.

Climate Studies: Impact of volcanic eruptions

Major explosive volcano eruptions, such as that of Mount St Helens in May 1980, inject ash and gas into the upper atmosphere producing aerosol layers which can affect the global energy balance and climate. Eruptions have been studied empirically and found to cause a decrease in the surface air temperature over the northern hemisphere land masses of up to a few tenths of a degree Celsius – an effect which may last for two to three years.

Climatic impact of explosive volcanic eruptions

Major explosive volcanic eruptions inject ash and gas into the upper atmosphere, producing aerosol layers which can affect the global energy balance and climate1. Empirical studies have shown that major eruptions can produce a decrease in surface air temperature of up to a few tenths of a degree Celsius over the Northern Hemisphere land masses and that the effects may last for 2 or 3 yr (refs 2–4). This temperature decrease has been simulated by numerical models using realistic estimates of the nature of the aerosol cloud1. Previous empirical studies of volcanic effects have examined fluctuations in monthly, seasonal or annual climate data, but generally only at a frequency of one observation per year. This has rendered determination of the timing of the onset of effects during the first year impossible. Using continuous monthly surface air temperature for the Northern Hemisphere land masses, we resolve the month-by-month development and decay of the initial climatic impact. In the case of Northern Hemisphere eruptions, abrupt cooling occurs during the first two to three months, which is more rapid than previously assumed.