IAMC-Documentation - User contributions [en]

Policy - WITCH

2026-05-29T09:35:28Z

Lara Aleluia Reis:

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When no policy is applied, the model is running a business-as-usual (BAU) case, which serves as a reference case with which the results with policy are compared.

WITCH can handle several types of policies:

* Quantitative climate targets on global temperature, on radiative forcing, on atmospheric carbon concentration, on greenhouse gas emissions or on carbon budget. In these case additional constraints are imposed in the model. The resulting carbon mitigation cost can be obtained in terms of loss of consumption or GDP.
* Regional policies, such as energy targets and emissions reduction targets. there are generally grouped into Current policies (current in-law commitments), NDCs (includes the emissions and energy targets submitted to the UNFCCC for 2030 and 2035) and Long-Term Strategies (LTS, includes many of the net-zero pledges). I can also run regional carbon budgets.
* Carbon taxes can be imposed directly in the model, by adding a cost on carbon emissions.
* A carbon market can be activated, with the possibility to allow an to trade of emission permits. The temporal dimension is also taken in to account with the possibility of "banking and borrowing" over time.
* Additional subsidies, taxes and penalty on energies sources can be introduced.

The model is calibrated for some predefined policies: "BAU", "stabilisation at 450ppm", "stabilisation at 535ppm", "radiative forcing target at 3.2 W/m2" and "radiative forcing target at 2.8 W/m2".

Spatial dimension - WITCH

2026-05-29T09:31:00Z

Lara Aleluia Reis:

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All countries are included in the model and, by default, are grouped into 17 regions clustered by geography, income and the structure of energy demand.

The regional disaggregation is now flexible and the model can be run on different model configurations, typically from 5 to 17 regions, or more.

[[File:WITCH default regional aggregation.png|600x600px]]<br />'''Figure 1.1: Regions of the WITCH model'''

The 17 regions are:

{| class="wikitable"
! Region
! Countries
|-
|brazil
|Brazil
|-
|canada
|Canada
|-
|china
|China, including Taiwan
|-
|europe
|Europe
|-
|seasia
|South East Asia
|-
|india
|India
|-
|indonesia
|Indonesia
|-
|jpnkor
|Japan and South Korea
|-
|laca
|Latin America and the Caribbean
|-
|mena
|Middle East and North Africa
|-
|mexico
|Mexico
|-
|oceania
|Australia and New Zealand
|-
|sasia
|South Asia
|-
|ssa
|Sub Saharan Africa
|-
|southafrica
|South Africa
|-
|te
|Non-EU Eastern European countries, including Russia
|-
|usa
|United States of America
|}

WITCH features any regional aggregations in the so-called coalitions. Some common coalitions are:

* regional coalitions : each region is mapped to a coalition containing only this region.
* world coalition : a coalition containing all the world regions.

Coalitions and regions interact with each others because of the presence of economic (technology, exhaustible natural resources) and environmental global externalities.

Temporal dimension - WITCH

2026-05-29T09:29:33Z

Lara Aleluia Reis:

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The first reported year is 2005. The time horizon is 150 years, with 30 period of 5 year time step.

Longer time horizon can also be run until 2300 to avoid any end-of-horizon effect, but 2150 is generally sufficient.

Results are usually reported for the period 2005-2100.

Carbon dioxide removal (CDR) options - WITCH

2026-05-29T09:28:11Z

Lara Aleluia Reis:

The CDR options modelled in WITCH are BECCS and Direct Air Capture (DAC), enhanced rock weathering and ocean alkalinity enhancement.{{ModelDocumentationTemplate
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Population - WITCH

2026-05-29T09:19:05Z

Lara Aleluia Reis: /* Population */

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== Population ==

An important driver for the emissions of greenhouse gases is the rate at which population grows. In the WITCH model, population growth is exogenous. The model base year is dynamic and it is usually updated to the last year of available data, (currently 2020). It uses the most recent estimates of population growth. The annual estimates and projections produced by the UN Population Division are used for the first 50 years. For the period 2050 to 2100, the updated data are not available, and less recent long-term projections, also produced by the UNPopulation Division (UN, 2004) are adopted instead. The differences in the two datasets are smoothed by extrapolating population levels at 5-year periods for 2050-2100, using average 2050-2100 growth rates. Similar techniques are used to project population trends beyond 2100.

== Economic growth ==

The GDP data for the new base year are from the World Bank Development Indicators 2007, and are reported in 2005 US$. We maintain the use of market exchange rates (MER). World GDP in 2005 equals to 44.2 Trillions US$.<br /> Although part of the GDP dynamics is endogenously determined in the WITCH model, it is possible to calibrate growth of different countries by adjusting the growth rate of total factor productivity, the main engine of macroeconomic growth.<br /> Economic growth rates and the level of convergence are strong determinants of energy demand and, therefore, GHG emissions. In the model, we depart from existing IPCC scenarios, and base our projections for regional GDP growths on assumptions regarding labour productivity convergence.<br /> OECD countries are assumed to reach a rather constant growth rate while the catch-up of non-OECD is driven by labour productivity which should bring most developing countries closer to the level of OECD countries by the end of the century. The convergence is nonetheless slow in per capita terms given the higher population growth of developing countries. Sub-Saharan Africa, in particular, experiences delays in catch-up. Eastern Europe shows the highest convergence rate. The model is therefore dynamically calibrated to match a growth path consistent with these underlying assumptions on convergence and growth.