我们先从冰河世纪开始讲，先不用着急

冰河时期的温度以末次冰盛期（约2万年前）数据最为详实，这一时期全球平均温度约7.8℃，较现代全球平均温度低5-8℃，且降温呈现明显的区域差异。高纬度地区降温幅度最大，北极、格陵兰北部等地温度比现代低14-25℃，部分区域可达-30℃以下；中纬度地区降温5-8℃，温度维持在5-10℃左右；热带地区降温相对温和，约比现代低5.8℃，仍保持在20-25℃区间。这些温度数据基于冰芯分析、海洋沉积物研究等科学手段重建，是目前学界公认的真实古气候记录。

全球温度突然下降20℃属于极端假设推演，其后果将是全球性的毁灭性灾难。气候层面，全球会瞬间进入极端冰期，中低纬度平原将出现数米厚的持续降雪，河流、湖泊及近海快速冻结，海洋冰盖向赤道扩张，大气环流系统崩溃，风暴、寒潮频发。生态系统将全面崩溃，农作物几乎全部冻死导致全球粮食绝收，温带森林大片枯萎，热带植物近乎灭绝，仅少数耐寒植物能存活；草食动物因食物短缺和低温大量死亡，肉食动物随之陷入生存危机，冷血动物基本灭绝，食物链彻底断裂。人类社会将面临秩序崩塌，粮食储备仅够短期消耗，饥荒快速蔓延；电力、供暖、交通等基础设施因低温瘫痪，物资运输中断，医疗资源枯竭，冻伤、感染等疾病无法得到救治，全球人口将大幅锐减，人类文明面临严峻挑战。

针对“末日温度降20度对南方恐怖、北方人习以为常”的观点，实际情况并非如此。北方人习惯的是渐变式的冬季低温，且依赖完善的供暖系统、除雪设备和稳定的物资供应链，而末日20℃骤降是无任何缓冲的极端低温，北方冬季常规低温会在此基础上再降20℃，远超人体耐受极限。更关键的是，极端降温引发的基础设施瘫痪、供应链断裂、社会秩序崩塌是全球性的，北方的抗寒优势会随供暖中断、粮食耗尽而彻底消失，并非“抗冻就能解决”。这场灾难的核心是全球生态系统和人类生存支撑体系的全面崩溃，与地域无关，不存在北方人“习以为常”的可能，本质上是全人类共同面临的生存危机。

The temperature data of the Ice Age are most detailed for the Last Glacial Maximum (LGM), approximately 20,000 years ago. During this period, the global average temperature was about 7.8°C, 5-8°C lower than the modern global average, with significant regional differences in cooling. High-latitude regions experienced the largest temperature drop: areas such as the Arctic and northern Greenland were 14-25°C colder than today, with some regions dropping below -30°C; mid-latitude regions cooled by 5-8°C, maintaining temperatures around 5-10°C; tropical regions saw a relatively mild cooling of about 5.8°C compared to modern times, remaining within the range of 20-25°C. These temperature data are reconstructed through scientific methods such as ice core analysis and marine sediment research, and are recognized by the academic community as authentic paleoclimate records.

A sudden global temperature drop of 20°C is an extreme hypothetical scenario, whose consequences would be a global catastrophic disaster. Climatically, the world would instantly enter an extreme ice age: mid-to-low latitude plains would see continuous snowfall several meters thick, rivers, lakes, and coastal waters would freeze rapidly, marine ice sheets would expand toward the equator, the atmospheric circulation system would collapse, and storms and cold waves would occur frequently. The ecosystem would completely collapse: almost all crops would freeze to death, leading to a global food failure; temperate forests would wither extensively, tropical plants would nearly become extinct, and only a few cold-tolerant plants could survive; herbivores would die in large numbers due to food shortages and low temperatures, followed by carnivores facing an existential crisis, cold-blooded animals would be almost extinct, and the food chain would be completely broken. Human society would face the collapse of order: food reserves would only be sufficient for short-term consumption, and famine would spread rapidly; infrastructure such as electricity, heating, and transportation would paralyzed by low temperatures, material transportation would be interrupted, medical resources would be exhausted, and diseases such as frostbite and infections could not be treated. The global population would drastically decrease, and human civilization would face severe challenges.

Regarding the view that "a 20°C temperature drop in the doomsday would be terrifying for southerners but commonplace for northerners," this is not the case. Northerners are accustomed to gradual winter low temperatures and rely on complete heating systems, snow removal equipment, and stable material supply chains. However, a sudden 20°C drop in the doomsday is an extreme low temperature without any buffer—regular winter low temperatures in the north would drop by another 20°C on this basis, far exceeding the human tolerance limit. More importantly, the collapse of infrastructure, supply chain disruptions, and social order breakdown caused by extreme cooling are global. The cold resistance advantages of the north would completely disappear with the interruption of heating and depletion of food, and it is not a problem that can be solved by "being cold-tolerant." The core of this disaster is the comprehensive collapse of the global ecosystem and human survival support system, which has nothing to do with geography. There is no possibility that northerners would find it "commonplace"; it is essentially an existential crisis for all humanity.