IPCC Climate Change Data: HADCM3 B2b Model: 2050 Radiation

The recent experiments performed at the Hadley Centre have
used the new Unified Model (Cullen, 1993). These experiments
represent a large step forward in the way climate change is
modelled by GCMs and raises new possibilities for scenario
construction. This experiment has overcome some of the major
difficulties that were associated with the previous generations
of equilibrium (circa IPCC 1990) and cold-start transient (circa
IPCC 1992) climate change experiments. HadCM2 has a spatial
resolution of 2.5 degrees x 3.75 degrees (latitude by longitude)
and the representation produces a grid box resolution of 96 x 73
grid cells. This produces a surface spatial resolution of about
417km x 278 km reducing to 295 x 278km at 45 degrees North and
South (comparable to a spectral resolution of T42). The
equilibrium climate sensitivity (DT2x) of HadCM2, that is the
global-mean temperature response to a doubling of effective CO2
concentration, is approximately 2.5 degrees C, although, this
quantity varies with the time-scale considered. This is somewhat
lower than most other GCMs (IPCC, 1992). In order to undertake a
'warm-start' experiment it is necessary to perturb the model
with a forcing from an early historical era, when the radiative
forcing was relatively small compared to the present. The Hadley
Centre started their experiments performed with HadCM2 with
forcing from the middle industrial era, about 1860 Mitchell et
al., 1995 and Johns et al., 1995. The greenhouse gas only
integrations, HadCM2GG, used the combined forcing of all the
greenhouse gases as an equivalent CO2 concentration. A further
series of integrations, HadCM2GS, used the combined equivalent
CO2 concentration plus the negative forcing from sulphate
aerosols. The HadCM2GG integrations simulated the change in
forcing of the climate system by greenhouse gases since the
early industrial period (taken by HadCM2 to be 1860). The
addition of the negative forcing effects of sulphate aerosols
represents the direct radiative forcing due to anthropogenic
sulphate aerosols by means of an increase in clear-sky surface
albedo proportional to the local sulphate loading (refer to
Mitchell et al., 1995 for details of this method). The indirect
effects of aerosols were not simulated. The modelled control
climate shows a negligible long term trend in surface air
temperature over the first 400 years. The trend is about +0.04
degrees C per century, which is comparable to other such
experiments. HadCM2CON represents an improvement over previous
generations of GCMs that have been used at the Hadley Centre
(Johns et al., 1995 and Airey et al., 1995). The experiments
performed have simulated the observed climate system using
estimated forcing perturbations since 1860. Johns et al., (1995)
and Mitchell et al., (1995) have established that HadCM2's
sensitivity is consistent with the real climate system. The
agreement between the observed global-mean temperature record
and that produced in these experiments is better for HadCM2GS
than for HadCM2GG. This implies that HadCM2Gs has captured the
observed signal of global-mean temperature changes better than
HadCM2GG for the recent 100-year record. The climate
sensitivity of HadCM2 is about 2.5 degrees C Like B1, the B2 world is one of increased concern for
environmental and social sustainability, but the character of
this world differs substantially. Education and welfare
programs are widely pursued leading to reductions in mortality
and, to a lesser extent, fertility. The population reaches about
10 billion people by 2100, consistent with both the United
Nations and IIASA median projections. Income per capita grows at
an intermediary rate to reach about US$12,000 by 2050. By 2100
the global economy might expand to reach some US$250 trillion.
International income differences decrease, although not as
rapidly as in scenarios of higher global convergence (A1, B1).
Local inequity is reduced considerably through the development
of stronger community support networks. Generally high
educational levels promote both development and environmental
protection. Indeed, environmental protection is one of the few
remaining truly international priorities. However, strategies to
address global environmental challenges are less successful than
in B1, as governments have difficulty designing and implementing
agreements that combine environmental protection with mutual
economic benefits. The B2 storyline presents a particularly
favorable climate for community initiative and social
innovation, especially in view of high educational levels.
Technological frontiers are pushed less than in A1 and B1 and
innovations are also regionally more heterogeneous. Globally,
investment in R and D continues its current declining trend, and
mechanisms for international diffusion of technology and
know-how remain weaker than in scenarios A1 and B1 (but higher
than in scenario A2). Some regions with rapid economic
development and limited natural resources place particular
emphasis on technology development and bilateral co-operation.
Technical change is therefore uneven. The energy intensity of
GDP declines at about one percent per year, in line with the
average historical experience of the last two centuries.
Land-use management becomes better integrated at the local level
in the B2 world. Urban and transport infrastructure is a
particular focus of community innovation, contributing to a low
level of car dependence and less urban sprawl. An emphasis on
food self-reliance contributes to a shift in dietary patterns
towards local products, with reduced meat consumption in
countries with high population densities. Energy systems differ
from region to region, depending on the availability of natural
resources. The need to use energy and other resources more
efficiently spurs the development of less carbon-intensive
technology in some regions. Environment policy cooperation at
the regional level leads to success in the management of some
transboundary environmental problems, such as acidification due
to SO2, especially to sustain regional self-reliance in
agricultural production. Regional cooperation also results in
lower emissions of NOx and VOCs, reducing the incidence of
elevated tropospheric ozone levels. Although globally the energy
system remains predominantly hydrocarbon-based to 2100, there is
a gradual transition away from the current share of fossil
resources in world energy supply, with a corresponding reduction
in carbon intensity.

英国哈德利气候预测与研究中心（Hadley Centre）近期开展的实验采用了全新的统一模式（Unified Model, Cullen, 1993）。此类实验在全球气候模型（General Circulation Model, GCM）的气候变化模拟范式上迈出了重要一步，也为气候情景构建带来了新的可能。本实验攻克了此前几代平衡态（约1990年政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）报告版本）与冷启动瞬态（约1992年IPCC报告版本）气候变化模拟实验中存在的多项核心难题。

哈德利耦合模式2（Hadley Centre Coupled Model version 2, HadCM2）的空间分辨率为2.5°×3.75°（纬度×经度），对应网格单元分辨率为96×73个网格盒。其地表空间分辨率约为417km×278km，在南北纬45°处缩减至295km×278km，等效于T42谱分辨率。

HadCM2的平衡气候敏感度（DT2x）——即全球平均温度对有效二氧化碳浓度翻倍的响应——约为2.5℃，不过该数值会随考量的时间尺度发生变化。这一数值略低于多数其他全球气候模型（IPCC, 1992）。

若要开展“暖启动”实验，需以早期历史时期的辐射强迫作为模型扰动输入——彼时的辐射强迫相较于当前水平相对更低。哈德利中心采用HadCM2开展的实验，以1860年左右的工业革命中期辐射强迫作为初始强迫（Mitchell et al., 1995; Johns et al., 1995）。

其中仅考虑温室气体的积分实验（HadCM2GG）以所有温室气体的综合强迫等效为二氧化碳浓度进行驱动；另一组系列积分实验（HadCM2GS）则在上述等效二氧化碳浓度的基础上，叠加了硫酸盐气溶胶产生的负辐射强迫。HadCM2GG积分模拟了自工业革命早期（本实验以1860年为起点）以来，温室气体对气候系统辐射强迫的变化。硫酸盐气溶胶负强迫的加入，代表了人为源硫酸盐气溶胶通过晴空地表反照率随局地硫酸盐负载量成比例增加所产生的直接辐射强迫（相关方法细节详见Mitchell et al., 1995），气溶胶的间接效应未被纳入模拟。

模拟的对照气候在前400年的地表气温长期趋势可忽略不计，百年趋势约为+0.04℃，与其他同类实验结果相当。HadCM2CON相较于哈德利中心此前使用的几代全球气候模型（Johns et al., 1995; Airey et al., 1995）实现了优化升级。

本次实验基于1860年以来的估算强迫扰动，对观测到的气候系统进行了模拟。Johns等人（1995）与Mitchell等人（1995）已证实，HadCM2的气候敏感度与真实气候系统相符。相较于HadCM2GG，HadCM2GS的模拟结果与观测得到的全球平均温度记录吻合度更高，这意味着在近百年的温度变化记录中，HadCM2GS对全球平均温度变化的捕捉效果优于HadCM2GG。

HadCM2的气候敏感度约为2.5℃。与B1情景类似，B2情景同样以环境与社会可持续性受到更多关注为特征，但二者的世界图景存在显著差异。

该情景下，教育与福利政策得到广泛推行，推动了死亡率下降，同时在一定程度上降低了生育率。到2100年，全球人口将达到约100亿，与联合国及国际应用系统分析研究所（International Institute for Applied Systems Analysis, IIASA）的中等预测结果一致。人均收入以中等增速增长，到2050年约达12000美元；到2100年，全球经济规模有望达到约250万亿美元。

国际间的收入差距有所缩小，但缩小速度慢于全球收敛速度更快的A1、B1情景。随着社区互助网络的完善，本地的不平等程度大幅降低。普遍较高的教育水平同时推动了经济发展与环境保护，事实上，环境保护已是少数真正的全球性优先议题之一。不过，由于各国政府难以兼顾环境保护与共同经济利益来设计并落实相关协定，应对全球环境挑战的战略成效不及B1情景。

B2情景的叙事脉络尤其有利于社区创新与社会革新，尤其是在教育水平普遍较高的背景下。技术前沿的推进速度慢于A1与B1情景，且技术的区域异质性更强。全球范围内，研发（Research and Development, R&D）投资延续了当前的下滑趋势，国际技术与专业知识的传播机制也弱于A1、B1情景（但强于A2情景）。部分经济快速发展且自然资源有限的地区，尤为重视技术研发与双边合作，因此技术进步的分布并不均衡。GDP的能源强度以每年约1%的速率下降，与过去两个世纪的历史平均水平相符。

在B2情景的世界中，土地利用管理在地方层面的整合度有所提升。城市与交通基础设施建设是社区创新的重点领域，这有助于降低汽车依赖程度，减少城市无序扩张。强调粮食自给自足推动了饮食结构向本地产品转型，人口密度较高的国家的肉类消费量随之下降。

能源系统因自然资源禀赋的区域差异而各不相同。提升能源与其他资源利用效率的需求，推动部分地区开发了低碳排放技术。区域层面的环境政策合作，在部分跨界环境问题的治理上取得了成效——例如二氧化硫（SO₂）导致的酸雨酸化问题，这尤其有助于维持农业生产的区域自给能力。区域合作还降低了氮氧化物（NOₓ）与挥发性有机化合物（VOCs）的排放量，减少了对流层臭氧浓度超标的情况。尽管到2100年全球能源系统仍以碳氢化合物为主导，但世界能源供应中化石资源的占比正逐步降低，能源的碳强度也随之下降。